1) Sometimes foreigners will be there, but it's mainly Japanese couples. If foreign couples, often interracial couples, where one of them is Japanese.
If you want to play with foreign men, better odds at Bliss-Out happening bar. Another thing you can try, if you are looking for other foreigners or foreign couples, is advertise on Craigslist, Tokyo Swingers, or AFF (AdultFriendFinder). You can also have the couple meet you at Olive-21, maybe after some coffee/drinks to introduce yourselves.
2) People are often in their underwear or lingerie, if they are not in the play area. A shower before play is the Japanese custom.
3) Olive-21 is for swapping and a Couple Kissa, so keep this in mind. If you want slow speed or gamble on nothing happening at all, then you would go to the Happening bars.
There are 2 main areas. The play area and then the couch and table. If you are
not ready or resting, you would sit on the couch. You can then talk to couples and decide from there.
If you are in the play area, you may be approached by couples. It is mostly a kind of
VIBE type of thing. It is hard to explain unless you have been there. Couples will get near you and then look at your face and body language for approval. If you give a negative hand sign, they will move away. If you give positive body language/hand signs or verbal approval, they will play with you. Keep in mind that negative body language and hand signs can scare away other couples around you, usually thinking you want privacy. You can also approach other couples too. Get near them and then look for approval.
Yes, you can also request verbal approval, but this can sometimes cause awkwardness. That's because the other couple can be playing. Don't worry or get hysterical about anything. The RULES of the club FORBID any kind of force or pushiness, and members are very respectful and polite. Interested people simply come near you and see if you will give approval.
4) Ages vary from 20s to 40s. If you are in good shape, very unlikely you will have problems. Just keep in mind that not every couple matches up or wants to play, but you should find many who will want to play.
5) Depends. There can be some extremely attractive women, and some good looking men (though Japanese men are usually of a slim built, as oppose to muscular, and of smaller size). Most people will be in good physical shape, particularly women.
6) Usually popular music. I don't usually even take notice of it
Your other questions:
7) Usually people are wearing something fashionable or sporty, not anything dingy in the couch area.
8) Yes. Staff is constantly at it.
9) Nobody can give an exact answer to this question. The best days are usually Friday and Saturday. Expect the place to be full or near full, during the evenings and night. Other days are less predictable, though Wednesday can be a good day.
Best if you go and find out. Just sit on the couch and have a beer/drink. You will often have to wait a little bit for the shower room to open, if Friday or Saturday, and you can talk with other couples.