I hate to say this man, but this post exposes a lack of understanding about this sport and event. Sometimes I get the feeling that a lot of people posting here have followed the sport for like 3 years or something.
Does Katir have shady connections? Yes. His agent is Miguel Mostaza who was the agent of Cacho, Estevez (two guys that took so many drugs they lost their hair) and Sergio Sanchez who was actually caught in 2013 after running 7.32 indoors and winning an indoor world silver back in 2010. He's been and is the primary agent for the top Spanish MD runners that especially in the 90's became extremely prevalent on the circuit after Barcelona (mostly due to their systemic doping program they had going on there that was basically exposed during Operation Puerto).
As for "Anyone with a positive test is of course more than suspicious" - well thanks for stating the obvious but of course "suspicion" can only apply to someone who hasn't tested positive anyway so let's be clear on that.
Of the view that 3.28 is a suspicious time? Well any time that fast raises eyebrows for a number of reasons and it definitely does when it comes from a guy who improves his PR from 3.36 to it - and yes it's significantly less when it comes from someone sporting a previous PR of say 3.30 even 3.31. I don't think you quite understand just how quick and difficult it is to run 3.28.
You talk about these "fall out of the sky" moments or "massive drops" - do you have any examples of these? I gave one - Rashid Ramzi. If you don't know who he is, then I suggest you google him. You mention a drop from 3.36 to 3.28 - how many exactly of these athletes over the course of history are there? I would challenge you to name them but I know the answer already. Of the 37 men that have broken 3.30 there are only 3 that ever made jumps like Katir did. There is of course Katir (3.36 to 3.28) and then there is Abel Kipsang (3.35.43 to 3.29.56) and Ronald Kwemoi (3.45.39 to 3.28.81). If you are putting Katir in the same category with Kipsang and Kwemoi then I think you really have a problem.
Is a drop from 3.52 to 3.40 suspicious? Short answer no. It's not implausible at all for someone with talent but mediocre training to do that. Also 3.40 for 1500m is a time that puts you 2257th on the all-time humankind list. Running under 3.29 puts you in a club of 16. So 12 seconds from 3.52 to 3.40 is numerically more than 3.36 to 3.28 but not even close in terms of relative difficulty and this is maybe the worst argument you gave.
Your final point - you think that if we studied guys that doped in 1500 you would see very little difference from their non-doped ability to their doped ability one year to the next? I think this was an inadvertently clever situation you framed up here because obviously doping impacts athletes differently and you could then simply point as say "well if it's not 8 seconds like Katir or 9 like Ramzi then it could be plausibly just natural progression. That's honestly an absurd claim or hypothesis and even if doping helps someone half a second, it's half a second they didn't have when they were clean. The time value is irrelevant but it's ridiculous to think that doping would actually ever make someone worse - that's not the point of doping.
Sorry to do that to you I really am, but it was a bad post.