Well, starting with the current version of moderncv you can have a look into the log file after compiling. There you can find, that style banking uses ...headiii to create the head for this style. Style classic or casual simply use another file ...head?. That is the reason that your first used command is not recognized, because it is simply not used in head 3.
With package xpatch you can change the relevant part of this file:
\usepackage{xpatch} % to patch the command ...
\xpatchcmd{\makehead}{%
\setlength{\makeheaddetailswidth}{0.8\textwidth}%
}{%
\setlength{\makeheaddetailswidth}{0.95\textwidth}%
}{}{}
As shown you can play with the number before \textwidth. The standard number is 0.8. That means use 80% of the textwidth.
Added this code into your MWE gives:
\documentclass[10pt,a4paper,roman]{moderncv}
\moderncvstyle{banking} % head3 body3
\moderncvcolor{blue}
\usepackage[utf8]{inputenc}
\usepackage[scale=0.85]{geometry}
\nopagenumbers{}
%\setlength{\makecvheadnamewidth}{15cm}
%\renewcommand*{\namefont}{\fontsize{30}{29}\mdseries\upshape}
\title{here comes the title here comes the title}
\firstname{My Name Is very}
\familyname{VeryLong}
\address{address comes here}{14000}
\phone[mobile]{00~00~00~00~00}
\email{mymail@gmail.com}
\extrainfo{0 ans Permis B}
\usepackage{showframe} % to visualize the typing area ...
\usepackage{xpatch} % to patch the command ...
\xpatchcmd{\makehead}{%
\setlength{\makeheaddetailswidth}{0.8\textwidth}%
}{%
\setlength{\makeheaddetailswidth}{0.95\textwidth}%
}{}{}
\begin{document}
\maketitle
\end{document}
with the result:
The used packages and version numbers to get this are (add \listfiles as first line into your code to get the list at the end of your log file):
*File List*
moderncv.cls 2015/07/28 v2.0.0 modern curriculum vitae and letter document class
size10.clo 2014/09/29 v1.4h Standard LaTeX file (size option)
etoolbox.sty 2015/08/02 v2.2a e-TeX tools for LaTeX (JAW)
ifthen.sty 2014/09/29 v1.1c Standard LaTeX ifthen package (DPC)
xcolor.sty 2016/05/11 v2.12 LaTeX color extensions (UK)
color.cfg 2016/01/02 v1.6 sample color configuration
pdftex.def 2016/06/17 v0.06h Graphics/color for pdfTeX
infwarerr.sty 2016/05/16 v1.4 Providing info/warning/error messages (HO)
ltxcmds.sty 2016/05/16 v1.23 LaTeX kernel commands for general use (HO)
colortbl.sty 2012/02/13 v1.0a Color table columns (DPC)
array.sty 2014/10/28 v2.4c Tabular extension package (FMi)
ifxetex.sty 2010/09/12 v0.6 Provides ifxetex conditional
ifluatex.sty 2016/05/16 v1.4 Provides the ifluatex switch (HO)
fontenc.sty
t1enc.def 2005/09/27 v1.99g Standard LaTeX file
lmodern.sty 2009/10/30 v1.6 Latin Modern Fonts
url.sty 2013/09/16 ver 3.4 Verb mode for urls, etc.
graphicx.sty 2014/10/28 v1.0g Enhanced LaTeX Graphics (DPC,SPQR)
keyval.sty 2014/10/28 v1.15 key=value parser (DPC)
graphics.sty 2016/06/02 v1.0s Standard LaTeX Graphics (DPC,SPQR)
trig.sty 2016/01/03 v1.10 sin cos tan (DPC)
graphics.cfg 2016/06/04 v1.11 sample graphics configuration
fancyhdr.sty
tweaklist.sty
calc.sty 2014/10/28 v4.3 Infix arithmetic (KKT,FJ)
xparse.sty 2016/05/18 v6512 L3 Experimental document command parser
expl3.sty 2016/05/18 v6512 L3 programming layer (loader)
expl3-code.tex 2016/05/18 v6512 L3 programming layer
l3pdfmode.def 2016/03/26 v6465 L3 Experimental driver: PDF mode
microtype.sty 2016/05/14 v2.6a Micro-typographical refinements (RS)
microtype-pdftex.def 2016/05/14 v2.6a Definitions specific to pdftex (RS)
microtype.cfg 2016/05/14 v2.6a microtype main configuration file (RS)
moderncvcollection.sty 2015/07/28 v2.0.0 moderncv collections
moderncvcompatibility.sty 2015/07/28 v2.0.0 modern curriculum vitae and letter compatibility patches
moderncvstylebanking.sty 2015/07/28 v2.0.0 modern curriculum vitae and letter style scheme: banking
tgpagella.sty 2009/09/27 v1.2 TeX Gyre Pagella as default roman family
kvoptions.sty 2016/05/16 v3.12 Key value format for package options (HO)
kvsetkeys.sty 2016/05/16 v1.17 Key value parser (HO)
etexcmds.sty 2016/05/16 v1.6 Avoid name clashes with e-TeX commands (HO)
moderncviconsawesome.sty 2015/07/28 v2.0.0 modern curriculum vitae and letter icons: awesome
fontawesome.sty 2016/05/15 v4.6.3.1 font awesome icons
fontawesomesymbols-generic.tex
fontawesomesymbols-pdftex.tex
moderncvheadiii.sty 2015/07/28 v2.0.0 modern curriculum vitae and letter header variant: 3
moderncvbodyiii.sty 2015/07/28 v2.0.0 modern curriculum vitae and letter body variant: 3
t1qpl.fd 2009/09/25 v1.2 font definition file for T1/qpl
ufontawesometwo.fd 2016/05/22 Font definitions for U/fontawesometwo.
moderncvcolorblue.sty 2015/07/28 v2.0.0 modern curriculum vitae and letter color scheme: blue
inputenc.sty 2015/03/17 v1.2c Input encoding file
utf8.def 2016/02/28 v1.1s UTF-8 support for inputenc
t1enc.dfu 2016/02/28 v1.1s UTF-8 support for inputenc
ot1enc.dfu 2016/02/28 v1.1s UTF-8 support for inputenc
omsenc.dfu 2016/02/28 v1.1s UTF-8 support for inputenc
geometry.sty 2010/09/12 v5.6 Page Geometry
ifpdf.sty 2016/05/14 v3.1 Provides the ifpdf switch
ifvtex.sty 2016/05/16 v1.6 Detect VTeX and its facilities (HO)
geometry.cfg
showframe.sty 2011/02/24 v0.1i showframe (new impl., RN)
eso-pic.sty 2015/07/21 v2.0g eso-pic (RN)
atbegshi.sty 2016/06/09 v1.18 At begin shipout hook (HO)
xpatch.sty 2012/10/02 v0.3 Extending etoolbox patching commands
hyperref.sty 2016/05/21 v6.83p Hypertext links for LaTeX
hobsub-hyperref.sty 2016/05/16 v1.14 Bundle oberdiek, subset hyperref (HO)
hobsub-generic.sty 2016/05/16 v1.14 Bundle oberdiek, subset generic (HO)
hobsub.sty 2016/05/16 v1.14 Construct package bundles (HO)
intcalc.sty 2016/05/16 v1.2 Expandable calculations with integers (HO)
kvdefinekeys.sty 2016/05/16 v1.4 Define keys (HO)
pdftexcmds.sty 2016/05/21 v0.22 Utility functions of pdfTeX for LuaTeX (HO)
pdfescape.sty 2016/05/16 v1.14 Implements pdfTeX's escape features (HO)
bigintcalc.sty 2016/05/16 v1.4 Expandable calculations on big integers (HO)
bitset.sty 2016/05/16 v1.2 Handle bit-vector datatype (HO)
uniquecounter.sty 2016/05/16 v1.3 Provide unlimited unique counter (HO)
letltxmacro.sty 2016/05/16 v1.5 Let assignment for LaTeX macros (HO)
hopatch.sty 2016/05/16 v1.3 Wrapper for package hooks (HO)
xcolor-patch.sty 2016/05/16 xcolor patch
atveryend.sty 2016/05/16 v1.9 Hooks at the very end of document (HO)
refcount.sty 2016/05/16 v3.5 Data extraction from label references (HO)
hycolor.sty 2016/05/16 v1.8 Color options for hyperref/bookmark (HO)
auxhook.sty 2016/05/16 v1.4 Hooks for auxiliary files (HO)
pd1enc.def 2016/05/21 v6.83p Hyperref: PDFDocEncoding definition (HO)
hyperref.cfg 2002/06/06 v1.2 hyperref configuration of TeXLive
puenc.def 2016/05/21 v6.83p Hyperref: PDF Unicode definition (HO)
hpdftex.def 2016/05/21 v6.83p Hyperref driver for pdfTeX
rerunfilecheck.sty 2016/05/16 v1.8 Rerun checks for auxiliary files (HO)
supp-pdf.mkii
epstopdf-base.sty 2016/05/15 v2.6 Base part for package epstopdf
grfext.sty 2016/05/16 v1.2 Manage graphics extensions (HO)
mt-ppl.cfg 2005/11/16 v1.6 microtype config. file: Palatino (RS)
nameref.sty 2016/05/21 v2.44 Cross-referencing by name of section
gettitlestring.sty 2016/05/16 v1.5 Cleanup title references (HO)
Update:
Having the name and the title side by side it needs a heavy rework of the building of the header, because that is only possible with using a tabular environment or simular ...
Because you want an easy way why not place the name in the first line, the title in the second line without delimiter. That can be done easily with the following patch (\makeatletter and \makeatother are needed now because the use of @ in that part of code):
\makeatletter
\xpatchcmd{\makehead}{%
\titlestyle{~|~\@title}%
}{%
\\\vspace*{10pt}\titlestyle{\@title}\vspace*{10pt}%
}{}{false}
\makeatother
With the complete MWE
\listfiles
\documentclass[10pt,a4paper,roman]{moderncv}
\moderncvstyle{banking} % head3 body3
\moderncvcolor{blue}
\usepackage[utf8]{inputenc}
\usepackage[scale=0.85]{geometry}
\nopagenumbers{}
%\setlength{\makecvheadnamewidth}{15cm}
%\renewcommand*{\namefont}{\fontsize{30}{29}\mdseries\upshape}
\title{here comes the title here comes the title}
\firstname{My Name Is very}
\familyname{VeryLong}
\address{address comes here}{14000}
\phone[mobile]{00~00~00~00~00}
\email{mymail@gmail.com}
\extrainfo{0 ans Permis B}
\usepackage{showframe} % to visualize the typing area ...
\usepackage{xpatch} % to patch the command ...
\xpatchcmd{\makehead}{%
\setlength{\makeheaddetailswidth}{0.8\textwidth}%
}{%
\setlength{\makeheaddetailswidth}{0.95\textwidth}%
}{}{}
\makeatletter
\xpatchcmd{\makehead}{%
\titlestyle{~|~\@title}%
}{%
\\\titlestyle{\@title}%
}{}{}
\makeatother
\begin{document}
\maketitle
\end{document}
you get the resulting pdf:
Does that help?
\firstname{My Name \newline{} Is very long}