Seems like a way to sidestep a confirmation process. Wouldn't need to put forward someone acceptable if you can just fire the current AG and install someone agreeable.

That might be why he picked a US Attorney (since they are confirmed by the Senate). I believe that the line of succession you are referring to comes from an Obama executive order, which Trump can change.

In any case, this will all be moot shortly (as the Republicans have the votes to confirm Sessions).

what do they have on Boente?

or it would have completely comparable to Nixon's Saturday Night Massacre. If you're old enough to recall, John Mitchell, the sitting AG, was forced from office. Elliot Richardson replaced him. Nixon told Richardson to fire special counsel Archibald Cox. Richardson refused and resigned. Then Ruckleshouse was sworn in as AG and he also refused to fire Cox and was himself fired. Then came Robert Bork who said he would and did fire Cox.