Mdd,
“Rambam says that if someone became a kofer and he is not a tinok she’nisbah (meaning brought up not frum), we as a community do not accept him be’teshuvah (even though Ha’SHem does, at least according to one pshat).”
There are many diff pshatim regarding this Rambam. One that I recall is, it only pertains to the 13 Ikrim and not anything else. Also, the Chazon ISH clearly differentiates nowadays as all have a Tinuk Sh’nishbu status .
In any case, in my opinion, as much as the two Dyanim can paskin other areas – a major Shayla like this – being that there is no clear psak and, Rambam’s as usual are complex with diff pshatim – a Halachik decision like this is above their head and should be consulted with major Gadolei Hador. Let it be submitted to the Eda Hacredis, Rav Sternbuch, etc. let them set up a meeting with all facts and experts on the table and then decide.
Perhaps there is a side that “Mutav ShyeChalel Shabbos Achod – shlo Yechalel Harbei etc. or goes into the issue of Chet beshvil Sheyzakeh etc. If the facts show that if you are mekarev them they will, and do come back then it certainly goes within the status of: you are permitted to be Mechalel Shabbos to save someone from Shmad. Certainly you can say that the above Rambam is permitted (which is not even a clear Issur DeRabanon i.e. associating with Avrayanim).
Many of these kids are emotionally disturbed and have issues in all areas of life as well, not only Yidishkeit. So yes, they are somewhat Cholei Nefesh” and thus a diff status as the above Rambam.