New kitten intros to older cats - what made a difference

[UnexpectedSwordInTheBin]

So we lost one of a bonded pair 2.5 months ago. The other cat was obviously missing his sister a lot, so we took the plunge and got a new kitten a few days ago. We were sold a girl, but fairly sure s/he is a boy (vet check on Thursday so can confirm then).

What we weren't expecting was for the older cat to be totally scared of the new edition. We've been scent swapping, and a couple of v brief meetings, but the older cat is totally freaked out. We have feliway plug-ins, and the new kitten is mostly confined to one room that the older cat wouldn't normally go into.
For those of you who've sucessfully done introductions, what one thing made a difference?
I'm so worries we've made a terrible mistake, and it doesn't help that I had totally forgotten how much work kittens are (2 speeds - sleeping or on the ceiling!)
Promise to pay in kitten tax in a few days... And thanks!

It's unpopular on MN but we've always just put the kitten in with the older cat(s) and let them get on with it. We did separate at night or if we were going to be out for long periods, but generally they were all fine with each other within 48 hours or so.

Jackson Galaxy advises introducing at meal times. Start way apart and each meal, move them a foot closer. They'll be focused on the food so will gradually get used to each others scent. That's the theory and it worked for us when our 3 siblings were apart too long after surgery and the girls hated the boy. He smelled funny, etc, although he recognised them. It took a week or so but they totally accepted him again. JG has a YouTube channel.

Thanks both.
Actually @coffeecupsandfairylights , I sort-of wish we'd just done that, maybe we'd be through the other side by now. I think part of the problem is I've probably researched too much... and ended up doing something hybrid, and now the older cat is scared of the new one! So we're trying to move forward in small steps. We have got as far as being able to hold the kitten and bring the older cat into the room. The older cat snuck a bit of food from the kitten's bowl this morning (in a different part of the room tucked away), and then ran away,. so hopefully that is progress...

I honestly don't believe you can do that much beyond making sure they all get to their food bowls okay and giving them lots of attention. They'll sort themselves out in their own time. I introduced two rescue 6-month olds to our 15 year old cat last year. They have plenty of space and just took their time working it out between them, though there was hissing and grumpiness.

All three of them are asleep together on the bed right now, and they groom each other, snuggle up etc. The old cat still gets a bit "get off my lawn, you whippersnappers!" from time to time, but I think that's some kind of dementia tbh. He'll hiss at them while grooming them sometimes.

That's promising - I'm sure you'll get there, honestly.

I do think we look too deeply into these things - let's be fair, most feral or outside cats seem to co-exist just fine without our intervention - yes, there's the odd scrap but generally they manage.

I just don't think they need us to interfere that much. Give them plenty of space and time and generally they'll work it all out in the end.