Can 12 AWG THHN/THWN wire be placed in a buried conduit?
Not sure where you got the idea that you need THWN-2, only.
All you need is the W.
TWN, THWN, RHW, XHW, XHHW, etc. are all perfectly fine, because they have the W that means they are waterproof, that means they can go in outdoor conduit. Most THHN is also THWN and MTW (multiply rated.)
If you are in a situation where the wire temperature may be extreme you might need to get picky for insulation temperature rating; but to be in exterior conduit (buried or not, they are all wet locations), all it needs is the W.