Why put guy no. 1 in the trunk?

Contrived?

Probably.

But no more so than:

1) insisting on using the same cab driver after they found you out
2) . . . and using the same taxi with busted windshield that will surely draw attention (one wonders why they didn't get pulled over yet again for that matter, guess they got lucky).
3) go to visit the cab drivers mother as opposed to simply making a phone call, which would have sufficed one would think.
4) rescue the cab driver from a crowded chaotic club when he could have easily found another way to get to his last stop
5) fail to shoot the cab driver (in the office building) when he had the chance.

I suspect it was part of Vincent's overall plan to have Max take the fall

He's implied to have done this before, when Detective Fanning or someone mentions the mysterious case of another cab driver who drove around and killed five people before killing himself.

Having the first body in Max's trunk sells the notion that Max, and only Max, was behind it all

That particular body was one of Vincent's targets. He didn't want THAT body left where it might be found too soon or the Federal Agents may have started scooping up the rest of the witnesses before he could finish the job.

There were things they could have done better. This movie doesnt drag as much as the first and most importantly it has one of the deepest mosy unsatisfactory ending. I love how theyre all giving up on their film at the end.