With my cupola I pretty much forced it off. I used the back of stanley blade and just eased my way around the lip until it gave. I managed to do it without damaging the cupola too, so the brute force option is always there.
Every glue release I've ever tried didn't work, at least not on the Heng Long tank. Heng Long seem to use poly cement that actually melts the plastic together, so glue release agents won't do much good. Be warned if you try to remove the frontal armour plate too. I removed mine in order to drill out the machine gun port for a metal replacement. It's not as thick as it looks and Heng Long seem to have used a whole tube of cement to attach the thing. It took about 2 hours to get it off and it cracked in the corner (thankfully I was able to repair with some putty)
The air intake pipes on the feifel cleaners aren't actually glued if memory serves, they are just jammed on with a run up. I cut mine off completely and shaved down the plastic stumps to allow the use of air pump piping, cut to length. It's pattern isn't to scale, but it's nicer than the original piping.