"2) some of the how and the why behind the Ssriel Llibani mob gaining the assistance of his Imperial 'guards'?"

A ritual sending a swath of plant-withering necromancy clear across a forest is something that requires a great deal of concentration - and also something that was likely to stir the Ancients and attract the not-so-happy attention of the Outlanders. Ssriel knew he needed protection.

As for why the Imperials agreed to help him, well, razing through a forest to make room for a useful and camo-free road is enough to make most Imperials smile. That's at least one reason, anyways.