Yes, but keep in mind that people use the short forms precisely to save time. In any case, both are equally right and common when talking about days of the week.
The first time i heard this sentence i thought thursday. I would think of fifth if it had a complement, liks position, for example
Weekdays are feminine in Portuguese, weekends are masculine. So you can only say "na quinta (-feira)"
You can say "quinto dia" (fifth day).
Technically yes, but in EP (where "quinta" is used for farm) chegar is always followed by "a" when talking about locations:
- Eu chego à quinta. I arrive/get to the farm.
- Eu chego à quinta na quinta. I arrive/get to the farm on Thursday.