Imacie wrote:
I've now used the Manta for the first time, was pretty impressed. I have to figure out how to make one for real. Two things I would like clarified.
1) if troops with tau jet pack disembark from Manta at end of a move to shoot with Manta can they reembark using jet packs?
Rule 1.7.5: Transport vehicles can pick up and carry units as part of the transport vehicle’s move. The vehicle simply moves into base contact with the unit to be picked up, and then carries on with its move as normal. Note that the transported unit is not allowed to move themselves during the move when they are picked up.
The pick up occurs on the vehicles movement phase, not the transported units movement, so they cannot reembark. 
Imacie wrote:
2) does the limitation of 4 armoured vehicles also encompass war engines? For example can I take 4 devilfish and then also take 4 riptides?
No, the idea from the fluff is that the entire cargo hold, lower and infantry levels, are converted to allow heavy suits. I'll change the wording in the unit notes to make that more clear. Thanks!