Yep that was the idea.
Lancer was close to Shirou and the circle and if it weren’t for Avalon coupled with Servant compatibility then the closest thing to a catalyst would have determined which Servant would be summoned. Plus it doesn’t have to be necessarily the spear itself. You could have Medb being summoned as a Saber using Caladbolg just because Cu himself was there and she sensed he was there.
Speaking of which here another one.
A no Avalon run for Shirou. Summons with affinity (most likely gets Siegfried). Let’s see how far he goes. And don’t forget the prior changes of a Shirou that doesn’t Avalon to help him when he uses magecraft.