how can I tell if its the shaft seal and how hard is that to replace?

I hear U - you never know how some one did things without having eyes, ears and hands there to actually witness it. I will agree about the shaft seal. Good luck.

How can I tell if it's the shaft seal?

it looks like you have changed everything but the seal,may as well replace the front bearing while you are at it.

Is there any way I can tell if the seals are bad? If it leaks in a particular place. How would I go about replacing them? Seems hard to get out and back in without damaging them or housing. I ordered another plastic impeller housing and am gonna try that first but am not optimistic.