Pointy-headed

Playing in a 25c matchpoints tournament on BBO, I picked up:

What would you open?  17 points, two aces and the spade suit. I decided to upgrade this to a 18-count, and open it 1S planning to rebid 2NT. Partner surprised me by bidding Jacoby 2NT and we landed up in the no-chance slam:

My soft diamonds should have dissuaded me and partner's 3 Jacks should have dissuaded him.

Two hands later, I was third hand and held:

Partner opened 1S.  What would you do? I decided to mastermind a little, knowing that in robot tournaments I would hold the best hand. So, knowing that partner had opened a 11-count, I passed!  Unfortunately, this was the full deal:

I averted total disaster by making 5 when I took a diamond finesse and East decided to cash the Ace of clubs and led a second club.

I should have masterminded a little more. Knowing that 1NT is 100% forcing, I should have bid it and passed partner's response. If the robot opened a 11-point hand, it would have been shapely and even a 2C response would shown real clubs.

Looking at the other tables, though, it appears that North rebids 2S (not 3S as I initially thought) and the Souths find ways to raise to 4S on that motley collection! What did those people know that I didn't?