Except no matter what path you take, you are willingly interacting with a malevolent being vastly more intelligent, patient, and experienced than yourself. It's an AI box scenario. If you are capable of interacting with the AI at all, it will eventually convince you to let it out of the box. If you interact with a Fallen at all, it will eventually corrupt your mind.
Except the AI in a box can't literally read your mind and isn't metaphysically superior to you on a conceptual level.
Yeah, but you can shove the AI box in your closet and ignore it. Ignoring something that has a (literally) demonic temper and the power to do almost anything it wants to you at whim? That sounds like an awesome way to wind up damned.
People keep assuming that once you pick up the coin you can just not interact. But that option is completely off the table. You will interact. You have no choice. At absolute best, trying not to interact amuses the Fallen and you wind up corrupted in a year instead of six months. More likely, while you are ignoring the single biggest chance for your short term survival as a human in Sunnydale, you'll become dinner for something a Fallen could have effortlessly saved you from.
Harry Dresden was able to seal away a coin by means of being a totally badass magical powerhouse, with exquisite training in magic. The SI is not Harry Dresden. He is ALREADY screwed. Nothing he can do will unscrew him. He is absolutely at the mercy of a Fallen angel. He can't run, he can't hide and he can't deal his way out. His only hope is to either hand the coin to a being powerful enough to deal with it (and even the Powers that Be probably aren't that strong) or he can hope that this Fallen will make his end merciful. He has literally no other viable options.
Making a good faith deal, with no attempts to weasel the deal, is the only thing he can do that has the tiniest chance of working, wi the resources he has at hand.
The thing about the Buffyverse is that there is no perfect safety. No one is safe. Glorificus was not safe and she was the safest individual seen in the entire series. Looking for safety is a fool's gambit in that universe. Or put another way: If you run you'll just die tired.
I just don't get the SI. He wants to live, but every time he gets handed something that will allow him to not only live but thrive, he reacts with horror and aversion.
I see something similar all the time in this one game I play (Planetside 2). Someone gets so fixated on the 'perfect' spot that they reject anything that is not perfect -- and as a result they die LONG before ever coming near the perfect spot, and usually one of the less perfect options they rejected would have saved them.
At the rate he is going, he will be dead or vamped long before he even catches sight of safety, let alone reaches it. Mostly because while safety does not exist in that universe, traps laid to draw in the unwary are plentiful.
Dress up as Harry Dresden. There is no way it could possibly go wrong.
Oh, plenty could go wrong. It's Harry. But on the positive side, when things do go wrong for Harry, it's almost always someone ELSE who winds up on fire.
Spell made you the wrong member of the trenchcoat brigade, you now have Constantine's literally supernaturally bad luck and ability to murder anyone by having them be an old friend who's helping him out just this once.
Quick! Befriend Mayor Wilkins! One stone, half a dozen birds.