Ah, I just abandon the idea of recovery on these phones and use the ubports installer once. Thereafter, I just flash directly. I have done the MTK tools dance, but don't any longer. Sorry :(

Ah, I have to admit I don't know much about porting :) We did put up a community wiki and the HADK is up there: https://sailfishos.wiki/books/hadk/page/hadk so that you might get some idea of the process ... I'm not sure to what extent this is different from that 'halium' builds that piggz makes. It makes sense to ask him directly, since he has a stream lined process that targets, among others, the phones you have.

As to why the hardware adaption layer is needed (ie. the android one) it's where all the binary blobs are :) Among other bits.