Split into your three points:
1. Service Interval
From Omega's FAQ:
"Like any high-precision instrument, a watch needs to be serviced regularly in order for it to work perfectly. Obviously, we cannot indicate the intervals for such work, given that it depends entirely on the model, climate and the care taken by the watch's owner. As a general rule, it is between 4 and 5 years, depending on the use of the watch."
So - general rule, rather than absolute - and taking into account the variable conditions mentioned above, your watch might have been ready for a service at 2 years.
2. "Performance is not the issue, build quality is"
Coming at this from Omega, or any other manufacturer of any device that is 5 years old, and which they have not touched since they sold it to you - they have no idea what you've been doing with the watch. None at all. It is out of warranty. One of the pieces of the case is missing, and the watch is full of water from a dip into a pool. They have nothing other than your word, that the pusher was there when you went in the water. They are under no obligation to side with you. They aren't even under an obligation to repair it for you at cost to you. They are 100% within their rights to say "sorry chap, you've bugger this beyond belief, tough". They won't, but they could.
Damage, or lack of maintenance is the issue, not build quality.
3. Swimming with a Speedmaster
If it's a speedmaster professional, the water resistance rating is 50 meters.
This, essentially makes it smash proof.
When brand new, with new, tested seals.
Not when 5 years old, with seals that haven't been changed, or waterproofing that has not been tested.
This does not make it suitable for swimming.
No one here would dispute that your situation sucks... but this is a case of user error, rather than manufacturer defect.
Edited by a mod Jul 10, 2021