League of Legends (LoL) Question: Rune page

It really depends on the champion, but I'll give you a safe one to default to and then you can branch out from there for different champions. Perfect Timing + Magical Footwear is also a good secondary tree setup if you want the Stopwatch.

I recomend learning the runes and what they can do, it will help you both to make your own rune page when you play new champion and will let you know what your opponend capable to do. Just try every tree page and keystones it takes about 20 games to learn 90% of the runes

That would depend on what type of mages you like to play. but it's possible. I don't play midlane much but after the first few runes and the secondary runes dont vary a lot if the champ play styles are similar. I do the same for my support. I have 2 pages and i make a few adjustments depending on who i pick and who the enemy picks.