So, apparently, the lavabending we see in LOK (Ghazan’s at least) is somewhat inspired by the bājíquán martial arts form which uses a lot of elbow and knee strikes.
As for Avatar Jafar and why he’s using something akin to waterbending form I can provide some possible explanations:
Avatar Jafar just has more bending/elemental knowledge (Even if the URN citizens know how to incorporate other bending forms into their style, not many benders in the URN stick to traditional forms. Beside, Ghazan and Bolin grew up in the EK/United Republic of Nations which is heavily influenced by the Hun gar/earthbending style of fighting. Although we see some airbending influences with probending, most URN bendrs seem to stick with an earthbending-influenced style).
Interestingly, Unalaq, the Water Tribe Twins, and Tonraq all seem to have an earthbending-based fighting style despite not being from the URN. I bet we can attribute that to cultural and environmental influences.
Bending lava from a volcano isn’t the same as bending lava out of the earth. The lava in a volcano flows more freely and does not need to be coaxed from the earth.
Going along with the second point, it could also be that the Avatar State just makes bending lava easier, and because of that Avatar Jafar can have an easy time using waterbending form to bend the lava.
I would have definitely loved to have people bending lava like a waterbender though.