In English, the tense of the main clause and relative clauses is usually relative to the time at which the sentence is spoken.
I waited until the bus came.
You use the past tense on both verbs because both the waiting and the coming happened in the past. But while you were waiting, the bus hadn't come yet! So, relative to the action of waiting, the bus ...
The Japanese language is based on relative tense. In your case, you have to choose みたいです ("seems") and みたいでした ("seemed") simply based on the time of your observation, but いない and いなかった are relative to the time of your observation.
= It looked like there was no one.
(You investigated the room a while ago and thought no one was there at that time....
Absolute times are ones that stay where they are as time moves onwards, or to think of it another way they're the ones that you can circle on a calendar or measure on a clock. So "Tuesday" or "May 25th" or "4 AM" are all absolute times, and the ever-changing now will eventually catch up to them and then pass them.
Relative times are ones measured in ...
(Disclaimer: I am a native Japanese speaker, but not an expert of language)
I feel there is a slight difference in meaning between (a) and (b).
Sentence (a) is the natural choice in most cases because of the relative-tense rule you have described. But (b) may be used to describe certain situations:
Other people ...
When going to Thailand, I studied a little of Thai.
Because 行く refers either to the present or future, it means that I studied Thai BEFORE going to Thailand.
When I went to Thailand, I studied a little of Thai.
This has the exact same meaning as in English. I went to Thailand, THEN I studied Thai.
抜けたら has nothing to do with past tense.
According to "A Dictionary of Basic Japanese Grammar", page 452:
"たら is a subordinate conjunction which indicates that the action/state expressed by the main clause in a sentence takes place after the action/state expressed by the subordinate clause."
If Mr. Yamada comes, I will go home.
An absolute time or date is a time period that will refer to the same time, regardless of when it is mentioned. These are typically exact times (on the same day in the current timezone unless otherwise stated) or calendar dates. In Japanese these are usually marked with the に particle but this can be omitted in casual conversation. For example: