Great idea! It would make them useful in the treaty.
With tolerance to the siege, they would not even need captured mortars. In fact they would be better since they also attack other types of units.
… And the coyotes with bonus and / or tolerance to the siege would make the Aztec army viable for the treaty. They don’t seem like drastic changes to me.
They could also have Apache allies as an infinite native card and enable the upgrade of ‘cavalry snail’ in the embassy to improve their reach by +2.
Also these suggestions so that the alliance with natives with firearms are just as competent as the native allies with Europeans.