I don't have any videos (or maybe I do; I should check.....), but I know from personal experience that I've combo'd nair into utilt many, MANY times. John12346 could probably vouch for that, if he came here, lolz. I've also kinda 'jank combo'd' nair into grab. I say 'jank' because I don't know if that's a true combo or not. You really have to take advantage of DDD's extremely fast falling speed, specifically his fast-fall speed, to surprise people with falling nair. It also comes out fast enough for you to surprise people above you during your ascent, and it lasts long enough to punish air dodge if timed well.
Jab 1 > grab and jab 1 > jab 2 > grab aren't actual combos and they're not even as reliable as other chars' jab > grabs. It's a mix-up that you can use, taking advantage of the fact that jab 2 also leads into jab 3, which can tack on a nice piece of damage. Therefore, you should be mixing up between jab 1 > grab, jab 1 > jab 2 > grab, and the full jab combo. However, tbh, DDD's jab is really a somewhat inferior move. I very rarely see top DDD's use it. If you're within range to jab, you're also within range to grab and DDD's grab is faster, both in startup AND cooldown. Jab is really just good for those 'uncertain' scenarios where you suspect the opponent might spot-dodge, IMO, or you're just trying to make him GTFO. In both cases, I like the multi-hitting jab 3, and I doubt I would actually go for a grab in those situations.
This is how dthrow on MK works. First, you have to read his DI. If he DI's upwards, you can re-grab him (or ftilt him, I think) if he does anything other than AD. If he AD's, you can re-grab/ftilt him after the AD. If he doesn't DI/DI's down and techs, you have to tech-chase. If he techs in place and you preemptively ftilt, you will hit him because techs leave you vulnerable during the tech animation's ending for a short period of time.
HOWEVER, ftilt > grab is not guaranteed. It's a mix-up that works on bad reactions.