Having previously worked shift work, I noticed that those who hooked up, but worked on different schedules, almost always broke up within 3 years, but the same wasn't true where there was overlap at home. I'd say that's because those on conflicting shifts almost never saw each other, and when they did, their body clocks were on different schedules / they were tired when the other was awake.
In terms of your wife cheating:
1. emotionally, sure, lets use that term. If we want to devolve 'cheating' down to non physical things, why not devolve it further and ask when was the last time you visually cheated? (looking at a beautiful woman / porn etc)
2. physically? Perhaps, but:
- you go into great detail about how naked her body was at the beach (most people stop at 'she was wearing a bikini'). You're obviously the jealous type
- you've snooped through her text messages, which were all platonic. But that doesn't reassure you, so you snoop again, and find them deleted. This to you, is a sign of cheating...but they were platonic messages
. Deleting platonic text messages speaks of not wanting a fight with a jealous husband
- not telling you he turned up to a night out...again, why would she? Dealing with jealousy is a no win situation.
- she had the opportunity for more with your best friend when you were asleep and everyone else was gone, but they weren't even kissing when you woke. No rumpled clothing either.
- hickey, on the shoulder? really? That's the area of choice? Well, it's possible, but the shoulder is also rounded, and bumping it while moving forward will cause a similar shape
- their friendship has blossomed while your sex life has suffered...are you sure you're sex life going downhill isn't purely related to: you letting your body go more & more, your conflicting shifts / never seeing each other, body clocks on different schedules, tiredness induced crankiness while the other is awake, and being turned down for being tired over and over, then giving up?
The above can be said for just about everything you've written.
Again, she may be physically cheating, but it seems to me that she's had plenty of opportunity for sneakiness where you would have caught her, and signs that say just the opposite (like purely platonic messaging).
I'd put it down to your work timetables conflicting so much. I do agree she's seeking her emotional needs elsewhere, as you're not there for her.
My guess is - One of you otherwise finds work that allows you time together, or it falls apart.