There’s controversy about how likely the vinyl cation is in this mechanism. At the bottom of the post there is discussion about a termolecular mechanism.

It is due to the electron-withdrawing nature of the halogens.

Well, if the internal alkyne isn’t symmetrical, then there would be two constitutional isomers (alkenes). Each of those constitutional isomers would be present as a mixture of stereoisomers (E/Z) since these reactions are not very stereoselective.

So four, at least.

What would the reaction look like for an internal alkyne, where there is equal substitution if the alkyne? How many constitutional isomers and products would be formed?

The reaction uses stoichiometric (not catalytic) acid.