Immigration/Customs at Frankfort

Pretty minimum for both. The line at passport check isn't long and moves pretty fast. Customs is non-existent or spot-checks only. You will definitely wait longer for your luggage to be delivered.

Frankfurt

as #1 posed baggage is the problem

count in about an hour

Frankfurt is an airport with much walking to do

The baggage problem also depends on the Terminal. I landed at Terminal 2 last month and was out of baggage reclaim within about 30 minutes.

#4
Depends also which company is handling the luggage. And other factors.

30 minutes waiting for luggage, 5 minutes for immigration, 10-15 minutes to get from Terminal 2 to the railway stations at Terminal 1.
Thus to allow one hour before train departure is a good idea.

> I'm trying to book my train & am not sure how much time to allow.
You don't have to book trains in Germany (with very few exceptions). You can reserve seats with far distance trains if you like.

with following tickets you have no problem to take one of the next trains. Maximum you can "loose" is the fee for the seat reservation (EUR 3,50).
- normal p2p ticket
- rail pass
- Rail&Fly ticket

you have to be careful with following specials, as the are only valid for the booked connection
- Saving fare (SparPreis) ticket
- Dauer-Spezial ticket
- Europe-Special ticket

The most basic advice I've learned (but it makes a difference) is don't stop to go to the restroom between when you get off the plane and when you get to immigration. Everyone always dawdles but if you head straight there the wait is next to nothing. Luggage is another issue, as is hiking to the train station...

Usually long distance trains run once an hour or sometimes even more often, so unless you've got one of these "must stick to a specific train "Sparangebote" you just go to the train station (there are two - long distance and regional/S-Bahn) and get onto the next one.