The path of the trailblaze allows her to continuously fight as long as she needs to without rest and are comparable to mydei and phainon who can fight nonstomp for 10 days straight. she also has 3 different healing abilities from 3 different path powers. Its very unlikely for her and saber to get weakened due to them targeting Stelle.
Aventurine 1-B is his final attack where he gambles everything he has into 1 attack. Its also not his starting move since his starting move is to launches dices and poker chips. I dont really see him using it anytime soon unless he really needs it.
Stelle can also taunt both of them and force them to both target her so Saber can fire excalibur.
Stelle also has abilities from the path of harmony, destruction, and preservation
I dont think Rho aias would be able to block a command spell amped Excalibur either since it was destroyed twice
Even when Shirou used it against Excalibur Morgan, he was extreme diffing it while Bellophron was offsetting Excalibur greatly