Does there come a point when those in the relevant services can simply ignore Trump and start briefing the incoming administration anyway?
No, not until Biden is inaugurated on 20 Jan. Scrotus is POTUS until Joe takes the oath. But I'd be willing to be there is more going on behind the scenes and under Scrotus' nose 'than are dreamt of in his philosophy'.
At some point can he be sued if he continues making unsubstantiated claims of election fraud? Surely he cannot continue to undermine the result? / Is it treasonous?
He has the same 1st Amendment rights as the ordinary American and it only spills over into treason or sedition if he actually incites insurrection. Innuendo and saying something isn't 'valid' isn't inciting, it's 'freedom of speech'. Baldly saying "Take up arms" or telling people to riot, disobey laws, or block everyday govt business, commerce, activities is.
When do the electoral college confirm the result? Do you think Trump will concede at that point? / according to google the electoral college meets on 14 dec to officially vote.
The election result isn't actually 'official' until the results are counted in a joint session of Congress on 6 Jan 2021 at 1pm EST. The votes are handed to the VPOTUS (Pence) and he opens and counts them before both Houses. If Pence refuses to attend the duty falls to McConnell. It should be interesting either way.
There are other steps before that day in the process. But no one has ever had to pay attention to all the steps before because they never really 'mattered' as once a POTUS concedes it's all pure formality, and the POTUS normally concedes as soon as enough states are 'called' to give the other candidate an uncontestable majority (270) electoral votes.
Personally, I don't think POTUS will ever actually concede that Biden won. I think he'll make a statement that he was robbed, the election was crooked, and he's being 'illegally deposed', then he'll either skulk out of the WH during the inauguration or he'll make a public exit a la Nixon which he'll call on all his cult members to gather and witness.