Learn English – How to join past continous and past simple with “but”

In sentence 1, I would prefer using past continuous (he was holding the hand ...), because using past simple will make it sounds like he held some white girl's hand habitually. (Note: If you change because to after, I think held will be more appropriate. It should also be even more appropriate than past perfect, in my opinion.)

To explain why using past continuous (or past progressive) is preferred, here is the closest entry I can find in Practical English Usage by Michael Swan.

422 past (2): past progressive (I was working etc)
3 past progressive and simple past: 'background' events
We often use the past progressive together with a simple past tense. The past progressive refers to a longer 'background' action or situation; the simple past refers to a shorter action or event that happened in the middle of the longer action, or that interrupted it.
As I was walking down the road, I saw Bill.
The phone rang while I was having dinner.
Mozart died while he was composing the Requiem.

In sentence 2, if you have this sentence alone:

Fans (queued/were queuing) overnight at a Hollywood music shop for the chance to get Lady Gaga's autograph.

I would say that either queued or were queuing can be used, depending on the way you want to narrate the scene. If you were a journalist writing it as a news, I think using queued would be more appropriate. However, if you were an anchor reporting the news, using were queuing would be more appropriate. This is because, according to my observation, TV news anchors usually use progressive tenses to arouse our attention, to make us feel as if we were in the scene of the news they are reporting.

However, when you gave the full passage,

Fans (queued/were queuing) overnight at a Hollywood music shop for the chance to get Lady Gaga's autograph. While they were waiting, Gaga saw their tweets. She immediately ordered 80 takeaway pizzas and sent them to her fans in case they were hungry.

I changed my preference to queued immediately. The reason is because it will provide the background (or the first reference time [ref.], if you prefer) for the whole story. Besides, there is already one use of the past progressive (... they were waiting, ...) as the main event of the whole story, which is sufficient for a news report.

For both example sentences, I agree with your book that "went" is correct and "was going" is incorrect. But why?

First, past continuous events aren't the main idea of the sentence. Rather, they provide the context for events in the simple past.

Second, if a simple past event happens "while" a past continuous event is happening, it always means the simple past event only happens after the past continuous event has started.

In the first example sentence, with "was going", it would mean that you left your parents while you were going, as in, you left after you had already left. This is impossible.

In the second example sentence, if both clauses were in the past continuous, there would be no simple past story, so the sentence would be about nothing other than describing a scene, which might be the context for some other simple past clause. This sentence appears to be about what Meg did, so it should be in the simple past.