Can a motorglider on condition be used for PPL flight training?
For non-commercial, non-AOC flying, yes.
Can a non type certified motorglider be used for PPL flight training?
Not sure what you mean by this. The aircraft is almost certainly an EASA aircraft & subject to the same maintenance requirements as any other privately owned, non-commercial aircraft.
Can a BGA instructor teach on a PA28 for trial lessons?
If he is also a JAA/EASA rated instructor, yes.
What changes will EASA have on all the above?
Very little.
they tell me they have PPL's and fly on a concession to allow them to instruct with a BGA, rather than a CAA instructors rating
It's not a 'concession'; the BGA administer NPPL SLMG instructor ratings with the approval of the CAA. You can get an NPPL SLMG from them which will be convertable to an EASA LAPL(A) for TMGs.
their aircraft are maintained by a CAA organisation rather than the BGA
The BGA is a CAMO & as such is subject to the same regulatory oversight from the CAA as any other maintenance organisation. They are required to ensure that every aircraft that they are responsible for is maintained according to the standards laid down by EASA in Part M.
You seem to have spent a lot of time listening to people slagging off BGA affiliated organisations so here's another point of view: commercial flying schools are run for the benefit of their owners, flying clubs are run for the benefit of their members.
Slopey is right. Go find somewhere that seems right for you & enjoy the experience.