As for Kelson executing Loris earlier (say, directly after the Interdict Schism), I don't think that would have been politically feasible. Loris' fanatical anti-Deryni beliefs were too widely held--if in lesser degrees--and the split in the Church hierarchy had to be patched up rather quickly so Kelson and the clergy could present a united front against Wencit. Executing Loris at that time might have made Kelson come off as a tyrant himself (Think public opinion doesn't matter? What of Jennan Vale?).
Of course, once Loris had sided with Caitrin, deposed and executed Henry Istelyn (especially in that manner, and after depriving him of the powers of his priesthood), then tortured and tried to kill Duncan, his death sentence was far more justifiable--it simply needed Cardiel and Duncan to agree to the handover (notice that didn't take long). I don't mind that it was done in Meara, either; the books tell us Mearan pretensions have been a problem for well over a century, so the hangings could serve to reinforce Kelson's authority--didn't the Mearans hang a Haldane viceroy in that same hall years earlier?