1) I don't know of any quick, easy ways to send contacts like the method you describe, but there probably are many. Whenever I need to send someone a phone number or an email address, I just type it and text it.
2) Plenty of my friends have iPhones (of course) and I've never once had problems receiving picture messages from them or vice versa. So no, there is no switch you'll have to flip to receive pictures from iPhone users.
3) There aren't really any big steps to hurdle in switching over. Android is pretty simple to use, especially Samsung's flavor of it. 4 year olds have phones these days.
Just in case:
If you do run into any problems upon purchasing your Galaxy S4, odds are you will not be the first to have had said issue (iPhone users switch to Android daily), so I would personally suggest you just Google your questions. That way you don't have to wait for a forum response.
I hope my poor answers did not discourage you from your decision to switch. The android community is very large and the marketplace is very vast, so there is likely an app or a youtube video for any reasonable thing you could possibly need to do (i.e. contact sharing).
"If you have an apple and I have an apple and we exchange these apples, then you and I still each have one apple, but if you have an idea and I have an idea and we exchange these ideas, then each of us will have two ideas."