Learn Russian: Эта квартира — наша собственность, в то время как у них ипотека. - This apartment is our property, whereas they have a mortgage.

No. While it can mean "while" in a strictly temporal sense (two things happening at once), here it acts as a conjunction of juxtaposition. It translates perfectly to "whereas," contrasting two states (our full ownership vs. their mortgage) without focusing on time.

I know а also shows contrast. Could we just use а here instead of в то время как?

Yes, you could completely replace it and say ..., а у них ипотека. The conjunction а is the everyday way to express this. Using в то время как simply makes the sentence sound slightly more formal or analytical, much like choosing "whereas" instead of "while" or "but" in English.

What are the punctuation rules for в то время как? Could the comma go right before как?

When it means "whereas" to contrast two clauses, the phrase acts as a single unbroken conjunction. Therefore, the comma goes before the entire phrase (, в то время как). You only split it with a comma (putting it right before как) in specific contexts emphasizing the exact time, which doesn't apply to juxtaposition.

Why is there a long dash between квартира and наша собственность?

In Russian, when both the subject (квартира) and its descriptor (собственность) are nouns in the nominative case, you use an em-dash (—) to bridge them. This acts as a replacement for the missing present-tense verb "is."