I asked about each of South’s three calls. In order, here is how they stand up. The double is fine. It is okay to overcall 1NT with a poor 18 HCP points but this hand has 19. A 1NT overcall would be an underbid.
Doubling and then bidding 1NT is fine, and this auction allows you to do that. When South bids 1NT, he describes a hand usually in the 18–19 point range. South has that range, and there is nothing about his hand to suggest bidding more than 1NT on the second round.
When North bid 2♥, he had learned that South had a very good hand. North’s 2♥ bid is a 100% weak bid. This could be the North hand:
♠ 4 3 ♥ 9 8 7 6 4 2 ♦ 7 3 ♣ 10 8 4
South’s hand has four top losers in a heart contract, and his side could have more losers than that if he is unlucky. South should pass 2♥ and
be comfortable about it. North knows that with 8 HCP points he could bid a game or at least look for a game. With a bad hand he can pass 1NT or can try to find a playable partscore in a suit if he has distribution.
Remember this rule: If you have a big balanced hand and your partner has nothing, the contract will play poorly for you. You won’t be able to get to dummy to lead toward your high cards. Instead, you will have to lead away from your high cards, which means it will be harder than normal to use your good hands. This adds up to a warning that when they open and you have a good balanced hand, it won’t be as good as normal.