Unhappy [Q] Can't install Mms.apk HTC One m7

hi everybody
I need your input!!

So, I am quite new in android world (been using iPhone for a while, but I am tired of their politics)

I recently rooted my HTC One m7 from France, installed a custom ROM MaximusHD 4.4.2, to use it in Japan with softbank
phone call is ok, 3G is working etc,
all went well, just stuck with Mms.apk
sending/receiving MMS here in Japan (softbank) is controlled by UA, so I wanted to install modified Mms.apk package found here

I first put the apk in sdcard, get in recovery (TWRP), installed it
it seemed to succeed, but when I boot back to System, Mms is not in the app drawer
I researched through RootBrowser, the Mms.apk does exist in system/app folder with the permission 644

then I installed through adb, but it says 'Failure [INSTALL_FAILED_ALREADY_EXISTS]'
so, I opened RootBrowser, erased the one I installed earlier, rebooted the device, and I have done the same thing through adb

then it goes : Failure [INSTALL_FAILED_VERSION_DOWNGRADE]
so, I used : ./adb install -r -d

then, next step, it says : Failure [INSTALL_PARSE_FAILED_INCONSISTENT_CERTIFICATES]

In case, I checked system/priv-app folder, and there is an other Mms.apk
I deleted it through RootBrowser, and even with ./adb uninstall in both locations (system/app and system/priv-app) etc,

nothing, it seems I can't install it
why?

the original files are from :
http://forum.xda-developers.com/show....php?t=2519733

or
http://forum.xda-developers.com/show....php?t=2641940

none of them worked
I would love to have a big help
I am going nuts

thanks a lot