Whirling Frenzy is a Class Feature Variant, which are “variant versions of several of the iconic class features common to the character classes,” and replace features rather than being a specific variant on an entire class.
So a half-orc paragon could select Whirling Frenzy, yes. The druidic avenger’s slightly more dubious, but most DMs allow that kind of “chaining.”
Ultimately, as variants, these things are entirely up to the DM, especially implementation details like who can or cannot have them. Most DMs I’ve played with have been very lax about it, and I personally think that’s a very good thing for the game (more options and variety is good), but you’ll have to talk to your DM.
For what it’s worth, I am aware of no combination or series of “chained” variants that will cause a problem, balance-wise.