Crime: What does this mean? 5% of my earned SIM could disappear, or something else?

Certain buildings can be robbed, Bank, Shoppping Mall, Casino. When robbed you loose out a share of your daily SIM income. Somewhere in the range 100 to 400.

Is 1st Edition going to be discontinued at some point in time?

Yes, should happen soon, although only gerber knows the exact date.
This game started quite unpolished, at the first month it was "played" only in discord.
The number of cards in the first edition is very limited.

Is it better to buy cards from the marketplace than gamble on spending 4 HIVE per random card?

This is basically your preference. Similar to Splinterlands, although I think here the cards from the house have decent value. There is no special cards going for crazy money from the house.
The most expensive cards are the high tier tech cards, that are given for free daily. The highest your education rate the higher the probability.

Am I right in saying that a high popularity rating attracts more citizens, including the homeless?

Yes, and you get mostly homeless :)

Is the level of SIM income dependent on your rank?

No it depends from your card collection. Each card has income, and the sum of those gives your total SIM income.
SIM is distributed daily.

My stats tell me I have a 3% chance of new citizens joining my city in 2h. Is that 2 hours, it’s always 2 hours and never changes!

Yes, every two hours. Although you will need to up your popularity for this.

At the end, yes its not very dynamic game, the main point is buying/selling cards and optimize your city. Make sure you have enough workers or not to much of it.

Also you can vote for president, who will then set your taxes, increase or decrease your SIM income, or set taxes that favor some buildings/cards, increasing their income :).