Hello Matt,
many sections of your story sounds very similar to my experiences I made within the last three month with my Disco 4 SDV6 and I also observe the P0105 MAP barometric circuit issue after MAP sensor exchange with TRISCAN after market replacement. Therefore I tried another unbranded after market sensor, but the DTC P0105 still remains. So I thought that this is common behaviour for after market products but the sensor in general is doing his job and is not the root cause for my the P1247 and P006A DTC's and the red rectangle of death in my display. But while reading the end of this thread I can imagine my assumption was wrong.
So if you have not suicides before you could write that your problem came back, I would be very interested exactly which MAP sensor you ordered and also, if allowed to write, where you have ordered it.

Thanks in advance
MauMan