Hey. Probably Runey will give you more detailed explanation as author, but I will try to give some answers too (because all replyes you can find in game story).
{SPOILERS BELOW}
1. "Defender of elf rights" - is just a mask for Cornwall. He created few minor laws about rights of elves and just took the whole slave market under his hand. He do not care about elves at all. Half-elves still do not have any rights and Cornwall is ok with this. His "slave ownership recall" possibility is just to deal with slavers he do not like.
2. Breaking in VERY old military facility. No one cares about this storage and there where just some human guards (yes, not even android - that shows how indifirent are politics about this place).
3. Yeah... here I partialy agree. MC is too powerless... just a normal human, not even very smart (because in story with High Elf Queen it was very obvious from the very begining that she do not have any "magic" powers and just uses fairys; or in Maria story - for me it was very clear that there is a high possibility for Maria to be a half-elf, I started to suspect this from the moment when I saw dark elf for the first time, in dungeon event with Cornwall).
4. Sylanar is society with slavery. DNA test sometimes is the only way to find that someone is a half-elf. Also, let me remind you that elves have some rights, but half-elfs do not have any. So it is very normal for hospitals to have some robots and jails to keep half-elfs when they find them.
5. When they find an half-elf that is accompanied with human they have all reasons to suspect that he is one who broke the law and secretely kept half-elf. So MC is very lucky that he didnt got even more problems with Maria.
6. Trenero do not have any reasons to help MC with slavery problem (he even have hundreds of slaves himself and do not care much if they die because of his actions). Cornwall have a strong antypathy agains MC and will never help him with something like this. Also he is in church of slavery in Sylanar, so he will just say "this is common event, just buy her".
I hope this helps.
P.S. Sorry for my imperfect English.