1) This is for Norway, check your country's AIP and NOTAMs for the relevant information; except in the Farris and Oslo TMA, the controller is responsible for correcting for low temps. To be on the safe side, don't accept an altitude you are not comfortable with.
If you do need to correct, you do need to request a new altitude from the controller, unless you are cleared for an approach. You do need to correct your minimas for low temp.
2) You should indeed correct minimum altitudes for low temp on a RNAV (like any non precision) approach. However be aware that for mandatory altitudes (specified to cross AT that altitude) you do need to either obtain a specific clearance for crossing at a different (higher) altitude, or stick with it. Compare to MSA to decide if it's safe.
If in doubt, ask ATC! But don't accept an altitude below what you're happy with
