1. Theoretically - yes (if the limit is imposed by lift available. Limit is more likely to be due to structural issues). In practice (legally) probably not. Athough I don't know for sure.
2. See above.
3. Dunno.
4. Yes. Check the MEL for your particular aircraft first though. Penalty would be slightly increased fuel burn (I'm told that winglets reduce burn by 7%) And aileron trim issues.