The answer to your questions A and B is probably
www.asiatravel.com as the previous poster has pointed out. They are very reliable and I have used them often.
Question C is 'Not really'. BKK is very safe for its size and Thais are generally friendly and helpful etc etc. You may not always find English speakers but your hotel should give you a card with the hotel's name and address written in Thai. That way you don't have to worry about whether or not your taxi driver speaks English (just whether he reads!)
Answer D is probably Yes, although you may want to consult guide books to see what you might prefer to do (culture, beach, temples, food) Very use I think is a day trip to Ayuttaya, the ancient capital. Call the Oriental Hotel and take their tour - it's the best. Go up there by bus (90mins) see the city, then the tour goes on the Bang Pa In, the Summer Palace. You go back to Bangkok on the Oriental Queen, a river boat that takes about 3-4 hours. You get a delicious lunch on board and just chill out with a cold bottle of Singha!