A few more thoughts - ok - so I get going to with a click bait title to try to get more attention. You got some folks of higher elo to comment, so mission accomplished. I just was a little sad thinking you thought I was trolling you (which to me, means I'm literally trying to screw you over or messing with you and feigning ignorance etc when my intention is to f with you). I think we sorted that out. And the issue is summed up well by you not liking my rune setup for various reasons and then really not liking it when I'm asking or questioning what you are up to. That 2nd point makes perfect sense to me and I get that will continue to try to not be negative, etc. I do want to address this in your OP though:
"Please trust me, convincing him that these runes have a negative effect on my ability to help us win the lane is not an option. More people than just me have tried to explain that Support runes do make a difference for the ADC's laning experience and he simply refuses to believe that."
Let's clear this up. My runes could have a negative impact effect on your ability to help us win the lane. The problem with your statement is that you and anyone that agreed with the premise on reddit and/or among our pals aren't really keyed into a very important factor - namely that it doesn't necessarily have a negative impact - and in my experience, rarely does. The only concrete scenario you could come up with where it would be factor is another crappy hypothetical. Let's say it pacov (gold runes)/thunder bot vs pacov (more standard support setup)/thunder bot with no jungle intervention. gold runes pacov has to play a little differently than non gold pacov which could potentially result in my adc getting zoned. Could. Or you can toss in the we're up against a substantially better bot lane (which I don't regularly experience and I doubt you do either).
All of that crap really boils down to how does gold rune pacov handle the lane - not the other 2 schmucks auto win lane regardless of their champs, skill level, chemistry. If the common outcome is loss of lane or adc is harmed, its a bad idea. As the guy running gold runes, I can tell you that's not typically what happens. Anyway, if it was clear that gold rune pacov harmed his team more often than helped, I wouldn't go gold runes. Not worth it then and obv a bad idea.
But summarizing - I recognize that gold runes could negatively impact your ability to win lane and I wouldn't use them if I believed it caused us problems.
I tend to think of runes/masteries kind of the way some old games work. You can pick from like 3 templates - 1 - good early, 2 - good mid, 3 - good late (obviously really simplifying). I believe I'm a good enough support that I can trade off some stats for that common rune setup and still control the lane... also gaining the benefit of added gold generation resulting in stronger mid/late game game.
Realistically, this enables me to be effective even if my adc is not. There's your selfish angle. It even enables me to do well for the team even if my adc is only doing so/so. But - if my adc is doing well, it means an early zekes for him and incredibly strong snowball potential mid game (including drake control). And the biggest thing, the trade off is generally decided in the 1st few minutes of the game.
Oh - and the enemy team being hyper aggressive because of some perceived weakness is generally a pretty good opportunity that can be countered in my experience. Being over aggressive tends to lead to less HP if properly handled... towards the aggressor. It varies ofc, but imagine a support walking up to wail on me as thresh. Pretty sure that enemy support will take a few shots from both me and my adc.
Anyway, I don't believe a gold rune setup is generally hurting my adc. If, even semi consistently, the result of a gold rune setup was hurting my adc, I wouldn't use it. Tell you what, though - next time we go bot, I'll run my setup and ask you to do just 1-2 things a little differently. Give that a go. We have a few games where you getting crushed in a row or hurt by me, I'll stop the gold runes for you, even though I don't believe its consistent.