1: Yes, unless you plan to smoke something stronger than cigarettes. Non smokers often complain, specially at Oase as ventilation there is rather weak in the main room. It is hard to find a girl at FKK that does not smoke.
2: Not at Palace. You can get it at World (think the prices are not all that bad, but I've never checked) and beer is served at some of the NRW clubs (included in the entry fee). Most girls at FKKs will not be all that thrilled if you smell of alcohol. FKKs are pretty sober places in general.
3: If by that you mean that you can go out and come back the same day, then yes they allow that. You must mention it before you leave. At Palace they will give you a stamped note you will show on re-entry.
4: Just about the same thing as in any European city. There are some nice restaurants in Frankfurt, there are some pretty good galleries, Alte Sacshenhausen area is great for bar hopping, there are quite a number of good night clubs, the RLD is ok to stroll through (save your spunk for the FKK though)...the list goes on. Hard to say without knowing what you like to do.
5: Well, worst case estimate is pretty relative, but I would say you would have to be extremely unlucky if it would take you more than 15-20 minutes.
6: No you won't.
Chivaz