I gave up on it very shortly after I tried it. I tried to make a platformer and the character couldn't jump half the time while it was on a platform moving down (as I'm sure the system detected that it was in the air, even if it looked like it was just standing on the platform). It also got glitchy when you tried to mix in physics with the character controller. This could easily involve some lack of knowledge on my part but it wasn't a very encouraging start.
According to their monthly survey, a third of all Steam users use 1920x1080 (increasing). Second is 1366x768 with 23.87% (increasing at half the rate). Whatever resolution you pick, make sure your support for those two is strong. I would personally use the first and scale down from there. Tile size, as everyone else is saying, is less of an issue. You can quickly make a mockup screen for each scenario (check this out as an example).
I suggest watching some Guacamelee gameplay videos. It's not swordfighting but it will give you a good idea on how decent animated 2D fighting looks. Notice that almost the entire frame changes every time. Dragon's Crown is another good example (here's the Fighter video, with sword). I know you don't have a team of artists at your command so you can only do so much but the more you animate properly, the better the game will feel, automatically. Also keep things in mind like characters being pushed back a little from damage and damage effects (like what Street Fighter also does).
Also, there are some decent animation tutorials on line if you're interested. Stuff like this. Google around to get some tips.
So you know, you don't need to know 3D math to create 3D games. Many engines like Leadwerks, Esenthel, Irrlicht, etc. don't require it and have functions to make your life easier (like calculating the distance between two 3D points for you via a simple function).
If your goal is really just to use graphics engines then I think you're doing more than you need to. For example, I use Leadwerks. Check out sample C++ code here which shows how to set up a scene, load a model and display it. If you understand that then you can make 3D games with it. Other engines are similar.
To be honest, you don't even need a language to start. There are good game makers out there like Construct 2 where you can start off with just changing settings and only later adding code whenever you feel it necessary.
That said, feel free to learn a language (I would suggest C++) if you really do have a lot of time on your hands.
It's a tricky balance but I think the safest bet is to look at other games in your genre and price bracket and see how long their gameplay is, if you can. People will naturally compare your games to others so it's good to know how yours stacks up anyway.
One very specific example of group failure is Hero6. They were around for many years but they never finished their game, despite people's best intentions. They accepted members from all over the place and they all lost interest over time.
You mentioned in two posts now that you'd like to make flash games. Do it! I know some C++ and ActionScript. I think you will appreciate Flash and ActionScript because the results are more immediate, so your learning is rewarded faster.
That said, you don't need to be committed to it to the bitter end. Focus on it for a few weeks and you'll know if it's for you. C++ has more options as far as engines and libraries you can use it with (I use a 3D engine called Leadwerks) but as people have said, there are no wrong choices to start learning.