1. dont want to speak with authority about the rules there but taxation on actual, personally-used phones seem asinine. i've also never ever once been scrutinized entering with phones for the 20 or whatever instances i've entered that country.
2. nothing special here with respect to means of spending. cash for convenience (e.g. small purchases) and cards for big purchases.
3. whether you need a guide for markets depends what is your objective in the market. if just walking by to see it, you probably dont need one. that said, i don't see their appeal at all: they're gigantic, crowded, and if you've seen a few markets like that around the world there's nothing awesome or outstanding about these ones.
4. the country has incredible food and a trip to the asian side is worth it to sample some restaurants. also, just the act of taking the ferry across the bosphorus is in itself a worthwhile thing to do.