1♣ is either 12-14 or 18-20 HCP balanced or unbalanced with ♣. 2NT would probably show the 18-20 HCP balanced, but your opponents are shooters. And, you partner is 75 years old :-)
The agreement is:
1x - pas - 1y - dbl = 14+ HCP
1x - pas - 1y - 1NT = other suits 8-13 HCP ...
I might have a different point of view of the signalling. I found out that people are obsessed with signalling. They do it till the end of the board, desperately trying to give partner an information, which can be deducted right away the dummy goes down. I heard discussion: "I ...
How about partner having
If I am playing with a regular partner, trust is the most important thing. Sure, cooperation is important, but my rule is: when I am asking for key cards, I should know why, otherwise I let my partner ask.
But, we can ...
Partner asked for key cards and signed-off, why should I think to bid on? If he does not have enough, he could have cuebided in ♥ or ♠ and let me ask for key cards.
I prefer to use 4♦ as BW5.5 in one way bidding.
I chose 5♥ as exclusion for ♣. But when I am thinking deeper about the hand ... I should bid 7♠. If partner does not have ♦A, I might go down as well from 5♠. Partner does not have ♥ values so should most likely have the crucial ...