1. Yes, if whomever has developed kmv player has a version that supports the 5800 (Symbian v9.4 with S60 5th Edition). As I don't know what the kmv player is, I can only suggest that you check with the developers of it, whether they have a Symbian/S60 compatible version or not. You can also look into the Nokia Store (Ovi Store) to see if there are other suitable video players apps for your phone.
2. No, as it would require changing the physical display and also adjusting the firmware (at least the display device driver).