To continue the story, eventually you have to complete all the missions. But no, you aren't forced to retry. You can exit and Freeroam for as long as you like without penalty.
GWHAHSHASHAA that's not my name. Curses fall upon thee C7. Yeah I move the weapon icon down so I can switch them easier...but I'm on an iPad so the default setup should be fine for you. Tap to aim at your target, otherwise the game prioritizes your enemies for you. This is a HUGE step up from GTA 3, where you had zero control over anything and just blasted away randomly. Of course, the PS2 & Xbox also had this issue. The controls are fine, the driving controls are stellar and work very, very well.
U mad bro? I mean it could just be because I played on a small screen, but I find the old GTAs to be very hard in general. And I'm pretty sure it's not just me. But whatever, I'm glad you don't find it difficult.
You're actually right, it's hard on the iOS. There were a couple of missions that I had to redo 10 times to pass. Controlling the RC and flying that airplane or something was freaking hard. Still, GTA vice city is one of my top 5 games on the iOS thus far