1. All three sentences are acceptable. However, the third one is more formal because its syntactic structure is standard "subject + predicate + object + 吗？". "有……(卖)吗？" is more common in oral Chinese.
2. "要" can be translated as want, need, must, have to, etc. depending on context. "Work overtime" is not a plan or a wish of the subject, so "要" here means "have to". If it is your plan or your wish, you can say "这个周末我又想要加班" instead.
3. "会" can be used to express possibility so you can say "如果我有很多钱，我(就)会去美国。"
Attention! Chinese grammar never includes tenses and moods, so Ada's answer has a logic mistake which "如果…… 就…… " is a pair of connection words（关联词）in Chinese rather than "would" will be translated as "就会" just due to subjunctive mood.
If you use "如果" you have to use "就" as well. "就" can be omitted.
I believe you must know literal translation and free translation. Obviously "Without a belt, the pants would be too loose." belongs to free translation.
"松" can be literally translated as "loose", but I guess "loose" doesn't always mean in a bad way in English. However, as a adjective, 松 usually means in a uncomfortable way in Chinese. (We use 宽松 to express loose and comfortable.)
e.g. 我感觉这条裤子有点儿松。(I think the pants are a little loose.)
这件体恤衫很宽松，我很喜欢。(This T-shirt is very loose and I like it very much.)
So it says "too loose" to translate "松" accurately.