Bridge & other card games/Openers Rebid withj 18-19 points
With a A5 AQ752 A64 K76 distribution I chose to open 1H planning to bid 2NT if partner responded 1S or 1NT. Partner responded 1S but then my RHO bid 2C! I bid 2NT to express my hand strength but my partner thought my bidding showed 12-14 pts. I thought that the overcall did not change my proper bid and 12-14 points is too weak to bid 2NT in this sequence. I did ponder the idea of bidding 3NT worried that my partner might not understand a 2NT bid and pass but 3NT would be too high if my partners 1S bid was based on 5-7HCP. Of course if I had not been so clever and opened 1NT we would have ended up in an easy 3NT contract. The hand made 5H or 4NT. Who is right and do you open 1M rather than 1NT with 17 HCP and 5 pieces in the major?
While it is not unanimous, more experts than not would recommend opening 1NT with ant 15-17 point 5-3-3-2 distribution regardless of whether or not the 5 card suit is a major. Some experts such as Marty Bergen and Larry Cohen feel strongly about this while others are a bit more equivocal. Especially if the major is hearts, opening 1H makes it easy for your opponent to overcall 1S while opening 1NT forces him to bid 2S instead, something he might be reluctant to do. Also, with a 16 point hand, if you open 1H and partner responds 1S you have no accurate rebid. Opening 1NT with this hand avoids this problem.
Having said that, even if you choose to generally open 1NT with these hands, a good 17 point hand certainly may be upgraded. The hand you had, 3 aces and a king, is certainly a very good 17 points and the aces and kings and lack of good intermediate spot cards suggest trying to play in a suit contract rather than in NT. Therefore I agree with your opening 1H with the intention of rebidding 2NT. If your hand was Kx, KJ109x, AQJ, QJ10, then a iNT opening might be preferable.
The auction you describe should have presented no problem. Your bid of 2NT did indeed show 18-19 points. Partner responded 1S and could have as little as 5-6 points. The fact that your RHO overcalled 2C does not add points to partner's hand. If he started with garbage, he is still looking at the same garbage. Therefore, if you bid 2NT with 12-14 and find partner with 5 or 6 you will probably be too high. If, instead, it was your partner who responded 2C, now a 2NT rebid by you could show 12-14. But that's a different auction. Partner shows at least 10-11 points.