Just got GTA5 so playing that at the moment, but I did have a quick look at the code. There should be literally no difference - talking to Harley moves you to the space to the left of him, makes you face upwards (if you aren't already) then activates a switch. Talking to Brianna directly, since yu're already in that space and facing up, directly activates the switch. I'll have to have a closer look obviously.
The difficulty curve is fairly constant (with, just like in my Lemmings levels, the occasional extra-hard or extra-easy thrown in); it'd be more accurate to say the 6th boss is the first extra-hard one. However, more than half of the bosses before him also serve as somewhat of a test that you're getting the idea of the mechanics than meant to be actual challenges (though Craig + Brianna have been known to give people trouble). It can also depend on your playstyle - for example, at one point there's two bosses more or less in a row, we'll just refer to them with the initals DR and DT for here. Those who stay relatively low-level and rely extensively on strategy will have quite a hard time with DR due to his combination of punishing you for piling on damage too quickly (which a higher level party is more able to handle) and having Auto-Regen. On the other hand, brute forcing your way through DT is practically impossible; if your strategy is really bad he can even take down a Level 99 party, though that'd require you to be pretty bad... more realistically though, a player who has some understanding but isn't thinking much, DT would probably beat their Level 50-ish party. (Intended level for that point is around level 30, though I've done both of them at only slightly over half of that).
I don't know of any dedicated RPG Makers for mobile OSes. Multimedia Fusion 2 is a general game-making program that these days supports most platforms for output (including iOS, Android and even Xbox 360), but it's not free - and the compilers for anything other than Windows are an extra cost on top of the program itself. It's also not the best suited to RPGs, though it's most certianly possible to make.
I'm guessing you don't use Facebook either? The page there is generally the best way to contact me. But if not, just post it here, I do come here at least once in a while even at the times when I disappear for ages (I often don't post for like a year, but I still visit a few times througout that) so I'll see it eventually.