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If you have a paper agreement to delete in exchange for your payment, that meets the criterion of a legal contract.
Unless they assert that you fabricated it, then it is gold.
They are not apt to go before a judge and assert any intent other than what is stated in their letter..... that they will deleted withni the reasonable period specified.
After receving payment, they have no business interest in attempting to defend a breach of contract suit.
I would think that letter would be fine for proof. As for the score leap you just never know. I had a collection ($560) that was just placed on my reports in Jan 2016 come off my reports today. I gained a whole 4 points on Experian. I'm not sure how it affected my TU or EQ yet since I don't get those until next week. You never know though you could see a greater leap of course but don't be disappointed if it doesn't happen that way.