Missing precision

I was playing with a non-regular partner in an online tournament, and we naturally decided to play 2/1.  We were having an excellent match (65-70%) until the last two boards.  This was board #7:



Is there any way playing 2/1 to avoid landing up in the slam?  The spade lead is dead-obvious on the auction too ... Playing precision, of course, it is quite easy to stop in 5C:

West

East

1♣¹

1♥²

2♦³

2NT

3♣

3NT

5♣⁴

Pass⁵

—

—

(1) 16+

(2) 8+

(3) 5+ diamonds 16-21

(4) 6-5 or 5-5 in minors, 16-18

(5) no extras

The last board was a doozy of a fix:



Who bids 3nt instead of 4H with a soft stopper in spades and 5 hearts? Our opponents, that's who! My duck of the spade was right (click Next to see the play) -- I was hoping to set the contract if partner has one more entry. However, that was not to be, and 3NT+3 was a near-bottom.  Turns out that 3NT+2 would have gotten us only 5% more matchpoints.