1.
王先生處
At Mr. Wong's place
Can I also say 王先生度 and/or 王先生便 with the same meaning?
﹣We use "王先生度" in general.
2.
晒 saai3
This character seems to mean "all, complete"… Is it ever used on its own, or only in compounds?
﹣晒 has several usages.
﹣1. As you said. When it uses with verb, it means "all, complete". E.g. 我地食晒啦。We have eaten all the things. (If you want to say "we finished eating", 我地食"完"啦.)
﹣2. Use as an adjective. It means "the sun is so strong"(negative tone). E.g. 今日好曬。 The sun is so strong today.
﹣3. Use as a verb. It means "bake in the sun". E.g. 拎張被去曬。 Take the quilt to bake in the sun.
﹣4. Nowadays people extend a new meaning from 2. When some couple did some things that makes them look very in love, very sweet, we say "好曬呀" for mocking them (in a good way). It means their love is bright as a strong sunlight.
3.
jat1 dou6 man4
One door
How is the "dou6" spelt as a Chinese character?
﹣一道門
4.
How do you write "zing6 hai6" (meaning: "only") in Chinese characters? The dictionary gives various options, and I am not sure which one is the correct one...
﹣淨係
﹣Look, be careful there are several similar words can refer to "only", such as "zi2 hai6"(只係) / "zi2 jau5"(只有) / "zing6 dak1"(淨得)... However they are not always interchangeable. If you are interested, I can tell you more.