I think Fisher evaded the shinigami for so long because he was clever, had good decoy methods to aid in his escape, and had the ability to hide his presence. If he were an ordinary stupid hollow, any old shinigami could have shown up and slaughtered him.

He's certainly stronger than your average hollow, but I wouldn't even put him on the same strength category as the "Huge Hollow" types. He's certainly not on the level of a Menos, power-wise.

As far as what happens to trapped/assimilated souls when a shinigami dispatches them? I would say ... the shinigami killing a hollow doesn't send its soul to soul society so much as it disperses its soul, and sends the dispersed bits back into the cycle. Once someone's a hollow, they're not going to the afterlife ... their individuality is going to cease to exist. Only their energy, the substance of the soul itself, will remain. I would suspect that assimilated souls would end up in the same sort of state ... so if Ichigo's mom is part of Fisher, it's unlikely that her soul is going to be separated and left as a distinct personality again, in the same way that it's not likely that Kaien's going to show up as a distinct personality. A shinigami's work is sending intact "plus" souls to soul society, and freeing unsalvageable souls (ie: hollows), returning them to the cycle in purified shreds.