Help understanding digging in

[Mr Grumpy]

No problem with the noob questions, It really helps to enjoy these games if you understand how the mechanics.

So just to recap, if an IP or trench is created any unit in that hex will get a defensive benefit as you say, if that hex is subsequently left empty then is considered vacant and the defensive benefit is halved to any unit that then occupies the hex.

The full defensive benefit can be restored if another unit makes another successful digging in attempt, however the fact that there has been a successful digging in attempt in that hex does not impact on making any future digging attempts more likely to succeed, in that you are mistaken. 

The engineers do need to be set to digging in, they don't improve the chances of a non engineer unit they are stacked with making a successful digging in attempt, but have triple the chance of a successful digging in attempt themselves.

In answer to your last question, once a successful digging in attempt is made any units that occupy that hex get the defensive benefit regardless of which unit actually made the successful attempt.

Hope that helps...........