1. No
2. Yes, in the one time "I owe you a beer" event, but choosing it may have very negative consequences.
3. To date, there is no 3rd heart available.
Bernard does not appreciate it if you use a Love Potion on him for sure.
Feliar and Xeroth both seem to appreciate it when you use a Love Potion, and everyone else seems not to care or remember.
That does not mean future updates may not include some sort of penalty for using Love Potions on any character, even previously, so proceed with caution.