Is there no way to make the Check in Nuspli "optional" by setting it to disable?
https://gbatemp.net/threads/should-nusspli-have-an-option-to-disable-the-free-size-check.645001/
Or could Wup installer or nuspli installers aren't pefect with forwarders?
Both, WUP Installer and NUSspli, don't do installations. Sounds wrong? Well, the eShop also doesn't do installations. Instead there is a program called MCP running on the Wii U. It's kind of a master process doing security relevant tasks and other stuff. This MCP does the installations: WUP Installer, NUSspli and eShop just tell MCP where to find the files, on which media to install them (NAND or USB) and other metadata, then they start a loop in which they ask MCP for the current state and process the reply to i.E. show a installation bar, react to errors and stuff.
So chance that NUSspli is the culprit is pretty low. Note that there is still a small chance: NUSspli changes tickets on the fly if needed. For example older Teconmoon releases had a small error in their tickets, so NUSspli fixes that in case it detects such a ticket. Do you still have the original files of the injector befor installing it through NUSspli? If so compare the tickets (just take checksums) before and after installation. In case they differ DM me.
A bug seems to be somewhere but I'm not sure of anything.
This bug is known to many people. It's the CFWs (Tiramisu/Aroma) itself. You even seem to have pinpointed it to the mocha payload now (pretty sure it's not the sigpatches module as that just does exactly what Haxchi and Mocha did but I'll compare codes again next days to make sure).
The bug is GCN (and maybe Wii?) injectors black screening, correct? So the Wii U reboots as it shoulds, gamepad shows the Wii logo but then the game won't load but you'll just have a black screen.
Sadly this bug has been reported multiple times to the CFW devs but they just say it is impossible that a Wii U CFW interfers with vWii... We clearly see it is possible (this bug itself is the proof, no?) and there are things like c2w which clearly connect Wii U and vWii side. In fact one of the CFW devs (Gary) wrote an Aroma plugin which changes vWii stuff (
https://github.com/GaryOderNichts/evwii ), so they know it is possible but oh well...
Anyway, normally the fix for the bug is to simply reinstall the blackscreening injectors. Not sure why this won't work for you.
//EDIT: To reinstall a injector correctly you have to uninstall it in Wii Us data management first or simply use NUSspli, it will uninstall before reinstalling automatically. Such stuff (this auto-uninstall or the on the fly ticket fixer) are btw what I mean when I sometimes make statmenets like "NUSspli fixes bugs you didn't know they existed".