You‘ve 7 points, no fit with partner's suits and 5 cards in heart that are not worth a rebid (as partner might not have any Hearts). 2♦ isn’t going to work as it would be 4th suit forcing, showing a strong game forcing hand. The best option is to rebid 1NT to show your weakness.

3) What's your rebid?

You’ve 7 points, no fit for partner's suits but a good long heart suit that’s worth a rebid (although partner might not have any Hearts). 2♦ isn’t going to work as it would be 4th suit forcing, showing a strong game forcing hand. Therefore, rebid 2♥ to show your weakness (when you rebid your suit you show minimum), which is likely to be the best part score.

4) What's your rebid?

Partner wants to play in Spades or Clubs, neither of which are good for you. Correcting the bidding to 2♠ could be the right option.
But, you’ve a very good Heart suit, which is why 2♥ is most likely going to be the best part score. Bid 2♥. Even though you’ve changed the suit at the 2 level, the fact that your first bid was 1NT shows you’re holding a weak hand but with a good long suit. In this case 2♥ is the same as if you bid 1♥ and later rebid 2♥; it’s just that you couldn’t bid 1♥ as partner opened 1♠.

5) What's your rebid?

Isn't 2♦ 4th suit forcing? Not in this case!
The 1NT bid limited the hand to 6-10 points, showing a weak hand, therefore 2♦ now is a natural bid, which shows at least 5 Hearts and 4 Diamonds (likely 5-5) and 6-9 points. You’re asking partner to either pass or correct to 2♥. This is what your partners hand might look like:

♠A543
♥6
♦QJ6
♣AJ543

Partner will then pass as 2♦ will clearly be the best part score.

6) What's your rebid?

2♦ will most likely be the best part score now. Even if you play better minor, partner's 1♠ bid shows they have at least 4 cards in Diamonds: With 4-3-3-3 they would’ve likely opened 1♣ (and the same with 4 Clubs and 3 diamonds). So, with only 4 Diamonds, their distribution is 4-2-4-3 or 4-3-4-2 or maybe 4-1-4-4. Either way, 2♦ is the best part score. By bidding partner's suit at the 2 level (even on the 2nd round), you show 6-9 points.

7) What's your rebid?

2NT shows 11-12 points, an invitational hand, and a stopper in the unbid suit (♦ here). Rebidding 3♥ would show 10-11 points and 6 cards in Hearts.

8) What's your rebid?

10) What's your rebid?

Correct Answer: Double

Don’t bid 1NT as you don’t have a Spade stopper. You also can’t bid 2♥ as that would show 10+ points. So Double (in this case a negative double), shows (6)7+ points and at least 4 cards in Hearts. On your next bid you should plan to bid 2♥ to show a long Heart suit and 6-9 points (which is the same as if you’d bid 1♥ and later rebid 2♥ - see Question 3).