I'm a little uncertain about your question. First of all I don't know who is Hank. Secondly the Vent Steam ability for example on Mei Feng acts very differently and it is not a Ht5 stuff. Probably you would like to ask about Howard Langston's Steam Cloud aura.
If this is the case, the answers:
a) Ht5 aura can give benefits to other models on a higher level. Say Langston is on the ground floor but beside him there is a model on a Ht4 building. If the later is within the aura or within 1" of the aura when he is targeted through it then it will receive the Soft Cover benefits.
No, the aura does NOT blocks LoS. Only Dense or Blocking terrain can do that.
c) Yes, but only in case of attacks. Models could also use the cover bonus in case they are within 1" of the aura and the attacker must track LoS through it.