It should be, if the WFM kit uses the standard two-peg back attachment system. Some do and some don't, the zowort doesn't but the Lfrith/Aerial/Rebuild/Calibarn do. It might be difficult to do the other way around because this kit has a pronounced collar that interferes with other big backpacks, but it can still mount certain option parts

Compared to the average for HGUC kits, VERY frequently. It's the title MS of its own series and the former has always been popular despite fluctuating fan opinions about the latter, and the release of the Freedom movie has solidly pushed that needle into the 'I love it!' half, so it's no wonder that it still basically flies off the shelf when you factor in that the kit itself is honestly phenomenal and one of the best HG molds of all time (there's also the fact that it's the only Destiny Gunpla mold that isn't a hand grenade like the RG or old enough to vote like the MG but still). If you're willing to spend a quite a bit more or a fan of the new movie you might also want to check out the Zeus Silhouette's 'Spec II' version, which uses the same base mold as this one with darker colors based on the Legend Gundam (as Shinn's way of honoring Rey)