1. Which way should i DI to get Falcon out of combo's?
falcon is very easily combo'd and has bad defensive options. honestly learn to play falcon like it's N64, just don't get hit and when you do expect to die for it.
DI is character and situation specific, if you know something can kill you DI out of a situation where you will die. if you are in a situation where a hit can kill you, use SURVIVAL DI. You get a feel for it as time goes on but yeah if sheik ftilts you at high % you're dead.
2. How do I make my Falcon more defensive?
dashdance more and jump around your opponent and don't attack, space yourself outside of their attack range and make it seem like you're going to do something stupid, but just do nothing. observing and waiting are great defensive tactics, but not always useful, especially if your spacing isn't great.
the best thing you can do IMO is solidify your in-shield game. Learn all the shieldstuns of moves that frequently get shielded (spacie aerials and shines, sheik tilts, marth's fsmash), learn the timing to immediately jump after stun, and know your OOS options.
jump (which implies wavedash, aerials, jc usmash)
shieldgrab
roll
dodge
upB
drop shield
wd OOS is extremely important for spacing, and aerials OOS to punish blocked attacks can be devastating.
3. Which stages do I want to ban vs...?
IMO vs Fox/falco/Sheik just ban FoD. The only character I don't want to ban fountain against anymore is Marth, I'd rather ban YS. Friggin' Scar's Story. Also if you know someone is going to cp like pokefloats and you're more comfortable on FoD just risk getting the random there and ban floats.
4. At which %s will dropzone work?
dropzone is flashy, not terribly useful. only attempt it if you know it's going to hit, a much safer thing to do is run off, fastfall, and dj back and nair. it will gimp their double jump if they try to use it, so most characters are forced to use their upB, and if it's predictable you can edgeguard.