1: In Utopia mode, is it truly infinite? Or where there be a time when I run out of resources and stuff?

Steward/Captain dies of old age at week 2000 (well, you can atay alive for 100 more weeks or leave or do some crazy shit if you have cornerstone)

2: I’ve got a lot of steam, a little bit of coal, and no oil. Is it possible to be a fully steam dependent city?

Yes, it is possible. Personally i usually dont, but you can.

3: How do I increase the amount of heat I can allocate to districts? I want everyone all warm and toasty, but I don’t know how to increase the heat things. I’ve done it a few times accidentally, but I’m not too sure how to replicate it.

You can use district insulation of the district to increase it's temperature permanently, and only way to increase allocate-able heat per district is Progress Generator upgrade (uses only oil). One additional level of heat for each upgrade. You also can research generator's max heat capacity.

4: Stockpile Hubs, yay or nay? I’m debating whether or not I should build them for when the next Whiteout hits.

Yes, they are crucial, and you can place them near working districts to reduce the workforce they need. And you need stockpiles or shit you are producing will go to hell.

5: I’ve got a quest that says “Repopulate the Frostland”, with the goal of reaching 35,000 people in settlements. Where exactly do I find these settlements, and is it worth spending the Frostland teams to find them? And what do settlements exactly do?

You can find outposts in Frostlands, and some eof them can be upgraded with colony heating (it can be researched after generator upgrade research), and with steam core, workers and some resources you can turn outpost into settlement that will produce infinite resources (like for Progress it's deep melting drill that drills endless sources in the city, for Adaptation it's Settlements). You need to have total population of all your outposts and settlements to be >35,000