RE: Security problem of air tank in fuslage
You will not take it with you after the checking desk. You will leave it at this point. Your plane will pass the security check as other pieces of luggages. It will fly in the cargo hold, not in the passenger cabin. There is no reason to refuse it. No problem for the security check.
However, security agent would refuse a LiPo battery if it is travelling with you in the cabin. In the hold, no problem.