been fiddling around with snake a bit today and i'll answer a few things that haven't seem to have been answered yet.
nessbounder, if C4 and d-smash mine are on top of each other, the mines explosion doesn't set off the C4s unfortunately so it would still just be one explosion. even if snake activated the C4s remotely at the exact same time i think the explosion radius is slightly different so it probably still wouldn't hit although i'm not 100% on that.
as for attacks setting off incoming grenades, ANY fire based attacks will explode the grenades so throwing them at charizard as he's doing fair and stuff = instant boom. samus fair? same thing. samus dtilt? kapow. so basically they're nice for immediate throwing at anyone who has a decent ammount of fire based attacks.
when you first pull out a grenade snake will enter a "grenade holding" pose where you can't turn around but can move back forward and jump, you have 4 options basically from here. strong toss, normal toss, weak toss and by shielding snake will auto drop it (yes if you're holding the grenade and someone comes in close and you shield thereby dropping it, you are not only protected by your shield but the shield also helps obscures the fact that the grenade is there). by dropping the grenade you can then pick it up and throw it and use it as you would a normal item (this also is true for the enemy)
i'm sure most people know that you auto-pickup items by attacking so if you throw a grenade at someone and then do snake dash attack to follow up, he'll pickup the grenade as he passes it so you can throw it a second time (which usually is around the time the grenade is ready to blow)
hmm what else... the d-smash mine can be blown up from people rolling through it or getting too close without them being hit by it, it's more of a thing for people to be thrown or whacked into than something to deter them from going to that area. it can be blown up by lots of projectiles but that's when you use your anti projectile nikita to stop all that rubbish. i have to say i'm impressed with the nikitas ability to pass through full charge samus shots and links arrows.
C4 seems to only explode when snake decides to detonate with another dwnB, but the start-up animation for the C4 detonator is really quite lengthy. you're only going to hit with it if the person hasn't noticed they're standing on the small C4 or if you whack someone in its direction then detonate.
grenades and mine d-smash set off each others explosions although it doesn't really help increase the explosive range much. snake seems to only be able to damage himself via grenades, C4 and the mine d-smash. the mine is much less useful and much slower to pull out than simply dropping a grenade on the ground via shielding to control the stage. the u-smash can be tilted slightly the same way you could tilt some f-smashs in melee. snakes upB is the worst thing about him as he seems to be very vulnerable to projectile spam when coming to an edge since he doesn't seem to autograb quickly like the rest of the cast >>