Help understanding digging in

[nhill40]

Pardon yet another noob question, please.  :)

I feel like the concept of digging in is an important one to fully grasp, but I don't think I'm fully there despite multiple scenario play-throughs.

I think I understand the basic concepts: a non-HQ unit which has not moved/fired that turn can opt to "dig in".  If attacked while in this state, they will defend at a penalized rate...but, based on a calculated percentage, they may also succeed in fortifying the hex, which will have a defensive bonus.  If the unit subsequently vacates the text, the hex still maintains its fortified status.  If another unit subsequently moves into the hex, it will enjoy some defensive benefit, but not as much as it would/will if it "digs in".  A unit that is set to "dig in" within a hex that had previously been fortified will have a higher chance of succeeding in the digging in operation.

So, that's what I think I have a good grasp on.  A few finer details that I'm much less confident in:

There is apparently a benefit to having engineers in the hex with you in terms of raising your chances of successfully digging in.  Does the engineering unit need to also be set to "dig in", or do they just need to be stacked along with the unit doing the digging in?

Also, let's say 2 (non-engineer) units are occupying the same hex.  One is set to "digging in", the other is not. At the end of the turn, it's determined that the unit set to dig in was successful.  The stack is subsequently attacked.  The dug in unit then enjoys full benefit of having dug in, but what about the unit who is just hanging out in the hex?  Do they get any benefit?