Novgorod starts with a quite okay fleet, so the only eligible rival typically should be the TO. In that case.... Well, I waited for them to be defeated by the scandinavian player, but you are right, its not possible to avoid it.
Fullfill it, you need those troops anyway (i went for 30k for the first war against muscovy).
Likewise, fullfill it, since you will be able to get a diplo 7-9 ruler quite easy and the strategy relies on low infamy so you can do it quite easily with one gift, further reducing infamy.
Do it, but make sure the country becomes an OPM in the peacedeal so you will get the right core through the diploannex mission
Tricky one, though i seldomly see that mission, its not THAT common