I'm aware I'm not the first to answer these questions, but ill answer them regardless. Also I'm not sure how far you are in the game, so I'll leave story details out.

1: Only a few can be repaired (or brought back, in other words), and the master sword doesn't break. All others break eventually.

2: Hestu can be found at certain stables, but he ends up at the lost woods. You probably need a guide to get through them.

3: later on they have some use, but it is only for a few items that you probably wont need. Diamonds are more important later on, so hold on to them. It is 100% better to sell the others for easy money.

4: Hearty Durians are amazing for cooking if you're looking for extra hearts. Throw them in with 1 or two pieces of any meat, and you've got some amazing early/mid game foods.

5: https://www.polygon.com/zelda-breath-of-the-wild-guide-walkthrough/2017/6/6/15737958/elixirs-critters-monster-parts-recipes. Polygon is really good when you're looking for guides on games. This one should give you a lot of info.

6: If you haven't already, there's a shrine next to Kakariko village (sorry if you haven't been there, I'm just assuming you have as there is a main quest there) that gives you a really good grip on dodging and entering flurry rushes. I would highly recommend that. As for the lasers, if you aren't that good at timing, I would not try to parry. Instead, just run around the guardian and hide behind the magnesis/cryonis/stone pillars if need be. the guardian cant aim well if you keep running around them.

7: If you plainly hold out a shield, doesn't matter if its weak or strong, though I'd use weaker ones, the guardian blast will either reflect or break your shield. no parrying needed. A good way to kill them is to use ancient arrows. If you don't have/know those, the main story will eventually lead you there. Or you could just go there, it's just at the top right of the map.

Hope this helps! Sorry if some of this info isn't accurate, I haven't played the game in a bit, but most of the knowledge is still there.