Sorry, I'm not totally sure about these but I'll give it a go.
1. The friendship meter has no set % rate to my knowledge. It will deplete faster or slower depending on growth stage and hunger level.
2. The best ways to refill the friendship are feeding snacks, playing games, and using the special items (eco fork, eco chopsticks, and eco shopping bag, shown in the special/rare list of the item box, obtained by going to the Donuts Park on the first of the month and picking up trash).
3. What do you mean exactly? The friendship meter does not affect your ability to visit the matchmaker or the park, if that is what you're asking. I don't think the friendship meter goes up when you visit the park, but I'm not 100% sure on that one.
3 days to answer or else...your wrath will be upon us!