1) Which school of jurisprudence do wahhabis and salafis follow?
Wahhabi or salafi are what one could consider "independent schools" of sunni fiqh, but they mainly based their osol al-fiqh (sources of jurisprudence) on those of the hanbali school. So they are Hanbali with (somewhat) independent views!
Wahhabi is a reference to people who follow Muhammad ibn Abdulwahhab's movement or view of Islam, which is the doctrine of Saudi-Arabia, however the scholars there tried around the 90's of the last century to get rid of this "bad" or "unwanted" attribution and declared themselves as salafi or followers of the salaf (as-salih).
Also note that some people who are more or deeper into hadith sciences called themselves salafi (refering to as-salaf as-salih) or athari (which refers to people who follow the traces of the earlier -highly respected- generations of Muslims).
Salafi and wahabi beside this claim in matters of faith ('Aqidah) to follow the way of the salaf (as-salaf as-salih) so they rely on wordly meaning of the Quran and the hadith in faith matters, like the attributes of Allah instead of interpretations of meanings or some logical or philosophical explanation etc.
Note that also some "Modernist" scholars like Muhammad Abduh, Jamal ad-Din al-Afghani, 'Allal al-Fassi and many others called themselves salafis even if their view of Islam is totally different as they wanted a reformation of Islam based on the roots (or a new understanding of the roots). Not following the roots -> the doctrine of the three first centuries as is the case of what we mostly define as salafi.
As you see the word salafi has different meanings and I'm afraid that even some of those who call themseves salafi don't know about them.
2) Can there be a hanafi-wahhabi ?
I'd say no however theoretically it could be that you'll find a hanafi who calls himself salafi, as he relies more on (sahih) hadith than on fiqh views of the hanafi school! Or he relies in matters of faith on the salafi view instead of the maturidi view (which seems to be common among the hanafi scholars).
3) Whether there is any sub-sect/rule of wahhabi which makes circumcision optional?
I don't think so as they rely on hadith and in many sahih hadiths circumcision is quoted. Note that the main view of hanafi isn't that different yes it is optional, but it's rather strongly recommended to do it!