Hm interesting, do note that PSVita Adrenaline does not install PSP plugins the same way as PPSSPP, try looking up how one can prepare game, not vsh, not pops, prx for Adrenaline
The plugin.ini is only for PPSSPP
On the other hand while the hooking routines for real devices are there, I have yet to test them... If you manage to load it check if the log file ms0:/PSP/GTRemastered.log was created on the PSP filesystem at least
Yes it is possible. On some controllers however android PPSSPP might detect for example "pad1.R2 + pad1.Gas+" while attempting to map "RightAn.Left" onto right trigger for throttle. If that happens, try mapping again by first holding down the trigger, start the mapping of "RightAn.Left" on the touch screen, then release the trigger so that it detects "pad1.Gas+" only
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The control name "pad1.Gas+" might vary depending on keymap file ended up being used on your Android firmware. The above screenshot is from my Backbone One PS edition version 1 with LineageOS 20(Android 13).
Meanwhile, note that the "Analog trigger threshold" setting on PPSSPP is for when one maps an analog stick on a real controller to let's say, the four face buttons on the PSP, how much the stick should move before triggering button presses. It is not for mapping analog controls to analog controls