How to troubleshoot to rule out backlight issues?

I have an ipad 2 a1396 which needed a screen replacement, a new wifi and a new flex cable.

Now I seem to have stumbled upon a backlight problem. I assembled ipad before applying glue to check if everything worked. But alas... the screen came on , but extremely dim and now nothing at all.

I cannot power it off or do the home+powerbutton trick, i see nothing and the only response I get is audio signal, when plugging into a powercharger or the computer. It goes "ding" , like "connected" . but still nothing on screen.

So dear community , what do I do to troubleshoot this thing , so in the end I will know 100 % it is backlight filter/fuse/whatever-ya-call-it ???

Will it help to remove EVERYTHING incl mainboard , so power gets unplugged ? and reassemble everything and try again ?

I am desperate and need to rule everything out , before handing in to expensive repair in other country (yes noone here does these, nothing with solder involved)

which parts do I need to check an reinsert, please advice, thank you

I am really hoping for a no solder solution !!!