no, you can use your uai for 2 years only. after that it's based on your uni grades.
as for applying mid-session. i applied last year mid-session. it's very hard to get in. you have to be really good. i didn't get my mid session offer. i found out that i didn't get it for 2 reasons: my uai probalby wasn't high enough, and also many courses don't have any vacancies for new students.
i do however know, that someone applied for civil midsession from overseas and got in last year mid session. i don't know how her grades were, but i know she's clever, and probably her grades from overseas were high.