to hear your accident with T420..... its a very nice machine...i have couple of T420s in use
Not sure what or how u using the TP (broken 420) .....
but as a FYI ... i have had few TPs (t60p + t61p) both these machines are kinda 'perm'tly' connected to EXTERNAL DISPLAYS for use. The teen's @ my house love it as they get to use the EXTERNAL DISPLAYS which is lots bigger than 14 & 15 inch screens. These laptops = also have STILL - AS "NEAR FACTORY displays" (IPS panels)....<<saves my IPS panels for lots longer>>
SO yes its possible to 'hook'up larger displays to TP's that have their screen issues ( use toggle F' keys ) ...or via Docking stations (like I have ) to BYPASS the laptops inbuilt screens.
I am not sure of that exect method you described (soldering to the MB etc) will work.... as am no MB EXPERT.... however I do did try that some yearrrr's ago(on a t43p) and that time that MB would boot up but mins later would SHUT DOWN with "over heat' alerts and logs reflected those kinda issues ........ so I gave up and connected a EXTERNAL SCREEN ,,,, and later TRASHED that laptop with a HAMMER !!
sorry not much good news ... but just sharing some old experiances and suggestions ...maybe others can help you .... as I know there are a lots of guys who like to do things @ MB level ......