Units don’t block line of sight. When attacking you have to declare a (valid) target unit. Only units with blast will attack every non-aircraft in the hex.
“On a sucessful cover roll, can the attacker resume his attack with the effected unit?”
I’m not sure what you mean.
Attacks go as follows:
Attack declares target and rolls appropriate dice, counting successes for hits, etc.
If the defender is in cover and takes hits he does a cover roll; on a 4+ THAT attack is limited to disruption.
Procced to the next unit.