PLEASE HELP: SA Background check- Am I screwed?

Hey all,

I have an offer from a MM bank for a SA position and am seeking assistance. For context on my situation, I have had 3 IB internships, 1 asset management internship and 2 formal jobs that will all be verified by HR as a part of their background check process. There is one additional position that they WILL NOT be able to verify. It is a research assistant role in the summer between high school and college that I left on my resume as an afterthought. I should have taken it off because (1) I seriously embellished the role, (2) it is entirely unrelated to IB and (3) it took place before I began college.

I did very minimal, ad hoc work for a research assistant role and never formally signed anything that would verify me working at the uni. I reached out to the guy I had worked under at the start of my bank's background check process and he said that he would not be able to vouch for me.

He said that I should not have the role on my resume (already submitted in the application months ago) and he IS NOT WILLING TO CONFIRM THE DATES or that I ever worked there. He said that he considers it untruthful to have that research assistant role on my resume. I am seriously concerned. He even went so far as saying that I didn't even work for him that summer... To my face, he said I was lying for putting it on my resume (wtf...)

With regards to the employment verification, I have 4 finance roles, 2 jobs, my grades and a criminal background check that will all check out perfectly, but how much of an issue will it be if I do not include this research assistant role in the employment verification (it will clearly not be verifiable in any capacity) despite the fact that it was in my application?

Do you guys think I am going to be screwed during my background check? I feel as though the role was so unrelated to IB AND before I began college so it will not be an issue, but I am not sure... VERY freaked out to be honest. The fact that he will say that I was lying about the role if asked to verify is frightening to me. I have to treat that research role AS IF I LIED ABOUT IT, because that's how my singular contact is treating it. Is this one unverifiable and (unreported to the background check company) role enough to outweigh my 6 other relevant/verifiable roles and clean academic + criminal background? I have worked so hard to secure this summer offer and do not want something like this to cause me issues...

Please give any feedback! thanks.