I personally like medium magnets because they keep the cube in place without making it difficult to start turns, but only if the cube is relatively stable. If the cube is floppy then I would prefer stronger magnets to offset the instability.
That's a good idea, I. think its best to get the hard things out of the way first so that the rest is easy. Also have you learned PLL yet? I would recommend that first.
I notice that you rotate during LL, instead of doing U moves to turn the top layer to the right orientation. It's much faster to do moves than rotations.
Considering your F2L is 15 seconds, I think that you should be able to get to around 25 seconds or so if you practice on your cross a little...