> May a unit be activated multiple times in an 'Activation' phase of a turn?
Yes. If you want to use multiple cards to "rambo" a unit, you can.
Initial playtesting may suggest that this need to be toned down, either by limited the maximum activations by making it harder each time, or by limiting the scope of the turn to a certain number of cards (fewer than the 52 activation current available). I'm imagining that eventually someone will play a joker to cycle their good cards back into their draw deck long before the 52nd activation, particularly as many cards (maybe 3 out of four) will be used as riders.
> Riders has me flummoxed. Uses the Suit Bonus? Still not getting it.
Yes, rules definitely need an example. Let's say you have a unit of marines, that need a 4+ to activate. In your hand you currently have a 8 of spades, a 4 of hearts and a 2 of diamonds. You can choose to play the 8 of spades to activate the marines, which will give them a move and 8 attack dice. The spade suit gives them the option of digging in, which (let's say) you don't want here. You can simultaneously play the 2 of diamonds as a "rider" alongside your 8. The value, 2, is ignored, but the suit, diamonds, is added to the activation card. You have now effectively played an "8 of both spaces AND diamonds". The diamonds suit give you +1 to hit, so now your marines are hitting a little harder.
You can add up to three "riders" in this way, and the effects of multiple duplicate suits stack.