The reverse clutch piston which in your case is made of plastic is stuck - (usually happens in Cuore) - overheats, warps and gets lodged.
The guy (pappu) you are talking about knows how to rebuild automatics; but his repairs are only as good as the parts he can find. As there are no new parts found for automatics in Pakistan your result will be highly dependant on the used parts you or he can find.
I rebuild autos too, and for best results I get full rebuild kits first then open up the transmission.
p.s. when you meet pappu - tell him that Zulfiqar (VW Golf) said hi