I am so sorry that you and your family lost Oliver. =( I hope that all of the happy memories will bring you peace and comfort.
I do think it's a good idea to put Taz in the spare room. It'll give him more space to call his own. And, I'll bet he'll be happy to have a window that he can look out of.
Sending you positive vibes and hugs your way.
ZQ
Never easy when you've given them a good, long life.
Looks like you have some good ideas already.
As for a buddy, might try an older kitty, who is known to be good with other cats. An older kitty is a known quantity, their personalities are pretty well set.
Though, as Miss Teia has shown, they are still very adaptable to new circumstances.