Opened one bottle and found something inside that looked like a coloured version of vallejo's Heavy Gel
Yeah, that's not "too thick", that's just broken. I'm with Obsidian3d, that paint has been stored incorrectly at some point in its life.
Maybe the Army Painter folks just didn't understand when you described the paint, quite rightly as too thick. They may not have a real customer service department or policy, just someone answering emails.