Well I can answer all those loopholes.
Toriyama gave information regarding 17 and 18 not long ago.
- Android 17 is the only android with unlimited energy, Android 18 was made weaker because 17's power was believed to be the cause of his insubordination
- Android 17 and 18 were human twins before being kidnapped by Gero, that enhanced them with cybernetics
- They are mostly humans and organic, the point was to allow Cell to absorb them (17 wonders why Gero made them since he could make 16 already without humans)
- Android 18's name is Lazuli, Android 17's name is Lapis
- Android 17 after being resurrected (he was one of Cell's victims), found himself a job as forest ranger, and has a wife, he has adopted her two children
- Android 16's model was based off the son of Dr Gero, who was a soldier for the Red Ribbon army that died in combat.
- Android 17 has visited 18 and met his daughter, a few times, during the various time skips
As for why he didn't help in the Buu saga, remember he doesn't know how to feel ki. So he couldn't have done anything and then the earth blew up.
As to why Android 8 was restored and not 16, easy, when they restored earth after it was destroyed, every material thing was restored too (cars, buildings...), that includes robots. But it only worked for when the earth was destroyed, not 5 years in the past.
16 had no soul, unlike 17 and 18 so he wasn't brought back like 17 when they wished that the people Cell killed gets revived.