Is it not more natural to say "... agus tusa?"
Yes, I would think so too. Perhaps it depends on region?
\Either way, I think it should be accepted, and if it is not, you should report it.
I think it has nothing to do with dialect but with a grammatical rule. Definitely an error here.
Yes, 'tusa' is both the emphatic and comparative form.
Exactly; I think the rule is you always have to use the emphatic form after "agus".