We don't know how he gets from the gunsmith shop to the grotto; I imagine he took the gunsmith's horse; he and the gang are apparently on horses when they ride into wherever it is Sibley is retreating through; it's fair to say that once Tuco has the means (ie. guns) he'll appropriate the necessary transportation.
Isn't he free of the ropes by the time he arrives at the town? I can't remember.
He has his hands tied and has the noose around his neck when Blondie stops the horse to get rid of him.Then Blondie makes his "70 miles" comment, leading us to assume they road 70 miles.
I think you just presented the irrefutable proof that nothing was impossible in Leone's West.