I used to do alot of model kits and I know most metallic paints will run under a sealer coat, even ones that were from the same company and should have been compatible (like Model Master, etc). Might not be the case here but I'd bet it would just from my own personal experience. I'd just leave it painted and call it a day.

 

Depends on the paint and the sealant. Spraying enamel or lacquer over acrylic, you'll have problems as the solvent in these will dissolve the acrylic paint. An acrylic sealant like Future over almost any paint type will be fine though. I've coated plenty of Tamiya paints (plain or metallic) with Future without any issue. If you can't get Future use an acrylic based sealant, probably a semi gloss to try and match Apollyons surface finish.

 

Liquitex makes a really good airbrush varnish as well. They have different sheens, but I generally prefer satin. I use them over metallic paints all the time. But yeah, the solvent in lacquer will destroy acrylic.