I made heavy use of Instance IDs since it was a quick hack. I don't recommend using this type of approach in any larger project. It isn't very easily readable, as you can see, and also can introduce problems when creating/destroying objects. In this case it was a controlled scenario so I used IIDs.
Repeat int(TextBox(1).Text) - This means take text in instance 1 of textbox (the second one), and convert it to an integer
System pick by evaluate Text2.IID>0 - This means pick every instance of Text2 as long as the IID is greater than 0. This basically means pick all Text2s except for the very first one.
Yes it is possible. You can set the slider bar value through actions in your events.
Didn't get around to size, as I said I didn't finish it. I got stuck on rotation - the rotation amount gets reset when changing the other values.
It is possible to get colored texts through events as well. Refer to the manual.
I always use debug sprites/variables as a sanity check to see if and when events run (by making debug sprite visible in an event, I know that event ran), as well as checking what values get returned by expressions (by saving the result into a debug variable)