Any meeting needs someone to chair it - this can either be the Pontiff or a conclave official of some sorts. Alternatively you could have the next church to take the role supply the speaker to call 'order', agree to put forth motions, count votes, etc. Share the joy...
As for excluding WIT, I figure they can say 'we didn't recognise Wallac Isiliviere as Pontiff, so didn't recognise his authority under the charter to decide what constituted a tithe, lacking any Pontiff we decided for ourselves' - they are big enough to be allowed to get away with it, but if the Conclave elects a new Pontiff they lose the excuse.
But the next pontiff will probably lower the tithe, or allow it to be part paid (allowing full payment in service leaves the knights very vulnerable - if we all did it for a season or two they'd collapse), the WIT then either pony up or cease to be members by default. The rest of us then have to vote on the effect of a default - suspension, ejection, public humiliation by announcing their expulsion? Hmm, I foresee consequences whichever way...
Robhan's view is if the Conclave knuckles down and cuts the tithe significantly to force the knights to shrink into impotence, he will ask a 'band of the willing' to make up as much of the shortfall as possible, of course that might impact on the Grand Maester's threat assessment, but then, those not paying him clearly have the means to defend themselves...
