Server maintenance made me type this twice... fml
1. As far as I know right now, DED has a decent chunk of cooldown lag. However, both Marth's DB and Roy's DED are faster when used in the air. Marth's floaty butt could short hop it into another aerial, but Roy's too heavy/manly for that. An idea though would be to use the first DED high in the air, or even offstage; follow it up with an aerial, up tilt, up B, etc (I haven't gotten anything to true combo in training mode though). Other than that, I would just go for all 4 hits.
2. It IS a true combo, but I'm guessing only at certain percents/characters/DI, because like you i've experienced people getting out. Someone should do testing on that (perhaps me lol).
3. I also like using neutral B. Its cooldown time is so stupid good, and it's funny when people get hit with it. I also like to use foxtrot dash-dancing (>, ><, <>, ><, so on) to look for a grab. Both Marth and Roy's dash-dance are really good; I'd recommend anyone playing either character to use it often.
Screw defense for a second, though. I have trouble taking the offensive! maybe its because I mained Marth :/. I find myself getting my nairs shield-grabbed a lot... any tips on offensive play?