Rust above Citroen Relay/Promaster door rail - adv

Hi all - This relates to an issue with my 2017 Citroen Relay which is also in various parts of the world a Ducato, Boxer, Jumper or Man Promaster. I bought my van back during Covid when there were hardly any to choose from so grabbed it while
I had the chance. Since then I've been through a couple of family tragedies and then welcomed a baby girl into my life,
so the conversion is still only halfway through. While it's been sitting on my drive I noticed some rust patches start to show above the stainless steel door sliding rail; I kept telling myself I'll sort it but it's now spread from 1 patch to several with signs of more bubbling under the paint. There's not much info to find online but some suggest it's catalytic rust caused by contact of two difference metals. What I'm confused about though is that this van is one of the most common vans on the road, so you'd think by this edition it would have been sorted. What's even more concerning is when you look at the rust patches up close it appears that there has been some repainting going on (hard to show in the photos but you should be able to make it out). Apart from this the van seems tidy and i can't see any other bodywork issues; the only other sign of rust is on the inside of the driver's wheel arch, but I'm assuming this can happen as it's a vulerable spot.
The van only has 36k miles; my guess is that it was part of a fleet that was then sidelined in covid and it sat around with not much use. I bought it from a puegeot garage and it's possible they just give everything a quick respray where necessary as part of their process as they have all the kit there. If need be I'm happy to not respray it as I was contemplating covering this part with vinyl anyway, but I was hoping some kind folks could help me with the following questions:

1. Have folks seen this before and does anyone have any thoughts on the theory that this is caused by 2 different metals
2. And if so does that mean I need to add some form of lining between the rail and body (sounss like a nightmare!)
3. If I remove existing rust with something like a dremel and cover with vinyl will that prevent the rust of spreading?
4. Is it suspicious that it looks like there have been paint cover ups there?
5. Does that wheel arch add any more suspicion or is that pretty normal?

Photos attached (please note there is muck in the gap between the rail and the van just because it desperately needs a clean, and there are water droplets on the van which may look like paint spray)

Thank you so much to anyone who has made it this far - I am a little stressed at what potential job/cost there is ahead!