For your first question, I’ll just throw in this from the discord about a similar question not long back:
For the second, it varies definitely. Broadly speaking, IF YOUR GOAL IS WOLLAR PROFIT (it might be xp or sp instead, which has different advice), then it is different by aircraft type. See the wiki’s per-aircraft section. Each page should include profit curves, so you can see what distance airports tend to be the most profitable. It isn’t precise, because it’s a line of best fit, but it’s pretty good.
Here’s a sample page for the A320 family: Aircraft - A320 Family
And here’s the main Wiki Directory