1) I had connected a source building to two different cities. When I realised that I didn't need one of the supply lines , and that it cost me extra money to have it, I wanted to detach it to one of the cities. But there are as far as I know no way to detach a supply line unless the game tells you that a supply line is unnecessary.
You are able to select a supply line by moving your cursor over the dotted line in the zoomed-out map or the road in the zoomed-in map. Once you have it selected, you can destroy the line by clicking the detach button in the right.
2) When I hover my mouse over a tab or a button, there is often no information of what that tab or button is for. Many people compare Hegemony with Paradox and Total War games. Both those games have a little info tip showing when I hover my mouse over a tab or button and I wish Hegemony worked the same.
We have tooltips over most of our tabs and buttons, as far as I'm aware. You do have to leave the mouse still for a moment before it will appear. Which buttons or tabs in particular have you found unclear?
3) I opened a window to read about something and couldn't close it after. If I opened other windows that window stayed above the other ones and made it impossible to use them for upgrading or getting information. If I had saved a bit earlier I could have reloaded the game. But as my latest save was quite some time before the issue with the window and I was in a certain situation I decided to carry on until my part of Etruria was safe and worked without any major threats.
Edit: I found another thread with the same issue as my number 3 above. If that happens again I'll use the advice the TS of that thread received. "Press F2 and write getwindowtop():getchild("TalkingHead"):close() and hit enter."
What window was it that stayed open, do you remember what quest it was part of? It would be helpful if you could tell us what it said so we could track down which one it was and hopefully fix it. Note also that you may want to run global.talkingheadstate=nil in the F2 window so that other windows can show up.