OK. I'm not with the FAA, nor do I have references for all of these
questions, but I'm going to throw my answers out anyway. (I am a CFI,
commercial land and sea rated pilot who owns an amphibian).

Can a CFII give instrument instruction in a tailwheel aircraft if the
CFII does not have a tailwheel rating?
Answer is yes. And he can even log it. But you can't go on a filed IFR
cross country flight unless one of you has an IFR rating.

Can a CFI give instruction in a Seaplane if he is not Seaplane rated?
Answer, I THINK, is yes, so long as the instruction does not involve
landing on water.
(This one is weirder than you might first think, because an Seaplane
can be considered in a seperate Class seperate from land plane).

Can a pilot who has a complex rating be PIC in an Amphibian if he does
not have a Seaplane rating. (Almost all amphibians are complex).
Yes, so long as he does not land on water.

Can a pilot who owns an amphibian that is no longer capable of landing
on the water get a flight review in the amphibian?
Yes, but it will not include privileges of flight in on water.

Is a pilot who flies an amphibian current for carrying passengers if he
does three landings in his amphibian on a ground runway?
Yes, but he is not current for carrying passengers if it involves water
landings.

Can an CFI instruct without a medical?
Yes, but he cannot be PIC.

Not SURE all my answers are correct and not sure anyone really knows
the answers, but they should be food for thought.