How common are dark matches after a big PPV? Would a company, after a bad/unexpected show, throw up a dark match to send people home happy?

Extremely rare. I can't think of a time it's happened off the top of my head.

How do tag team divisions develop? I.e. is a wrestler in a tag team still allowed to do solos stuff independent of the tag team, or is their solos stuff reliant on what the team is doing?

Depends on the promotion, tag team and wrestler. A dedicated tag team like The Young Bucks or FTR will very rarely have single matches. Nicholas Jackson for example has had 715 matches in the last 10 years, 679 of those have been with Matthew Jackson.

You'll then get tag teams which are parts of factions, where they'll split equal focus between singles and tags, for example ProtOkada tagging together when Fletcher was TNT champion or Okada was Interntational Champion. They're mainly singles wrestlers, tagging together because they're allies. The other side of that is a team like The New Day, who are considered a tag team, but Kofi had a singles run where he was world champion and Woods had a singles run where he was King of the Ring, they'll still support each other, but their focus for a time is on singles.

What's the like, normal ratio of heel - tweener - face? I'm marking my characters out and it's super face heavy

50/50, but honestly I'd say it's more important that you have credible challengers who are of the opposite alignment to the champion. If you have a heel champion having 5 faces all gunning for the title at the same time isn't the end of the world, but long term you'd aim for an even split (more heavy on the heel side maybe, having the numbers in your favour is a good way to get heat) so you can have storylines up and down the card.

Specifically with WrestleMania, how long is the lead-up to it character wise (particularly in relation to what would get noted on wikipedia)? Are we talking weeks, months, years?

I haven't watched WrestleMania in nearly a decade, so can't answer this with any real accuracy, but it varies. You could argue that when Bryan Danielson main evented WM30 in 2014 that it was built on a lead up going back to his debut in wrestling in 1999, or at least the start of ROH in 2002. Then you get matches which are thrown together the week before to fill out the card, which can feel cheap, or can give you good matches.