Oh @pizzaandcats, I'm so sorry you're going through this. I don't have much to add here as it's mainly been said, but this drip feed for you must be incredibly hard. You must be constantly wondering 'what next'?
It reminds me so much of my exh (note ex) - the drip feed over YEARS about an innocent 'work friend'. From simple coffees at work and other 'innocent' meets, it ended up with him admitting to sleeping with her in OUR BED (and even that was the tip of the iceberg). But apparently that was okay because he'd changed the sheets afterwards. And he still made damn sure that I felt like the crazy one.
Regards the seemingly-innocent gift he came home with from her - there may be another gift(s) that he hasn't shown you? Something he left in his car, or fit in a pocket, or he hid well away - or left at the office. The glass engraving wording isn't exactly innocent though, is it, depending on what the 'inside joke' is? And if a 'work meet with a few colleagues' has now changed into a 'date-style meet for two with thoughtful gifts exchanged' it makes me wonder what else there is. I'd be thinking their 'lunch' was actually food ordered into a hotel room etc etc. But I admit, I'm old, jaded and don't trust anyone now.
The necklace... I know most people don't like it, but perhaps he chose it to suit her 'style'. There were plenty of other sunflower necklaces available, but that one was just a bit 'different'. To suit an 'in joke' or 'intimate moment' they've shared - or an outfit he's seen her in. I bought one of those diaries once for a dear female friend... it was a very personal gift and totally different in intention to a bog standard 2021 appointments diary. Not cool here. What's she going to write in it, memories of their illicit encounters so they can look back over the years and laugh at how they got away with it? And I'm not sure you stated how much the chocolates cost... I've been given a five quid box of Ferrero Rocher... and I've also been given a selection of individual handmade chocolates from a chocolatier (the latter from a guy and costing a whole lot more than a fiver).
Showing you the messages... I had this. Skimming past bits he didn't want me to see, making me feel crazy for even asking. By the time we got onto his phone, it was virtually blank. Everything deleted. Your 'D'H has now had plenty of time to erase the evidence. Have you seen his whatsapp, texts, Messenger etc? But the moment has passed. There won't be anything there now.
The upshot here is - if it feels wrong, then it IS wrong. To me, it's clear he's making you out to be the unreasonable one by even questioning his intentions. Damn you for forcing out these 'innocent' details from him. My guess is that he's trying to either give you enough info to 'appease' you and get you off his back (and if he gives you something you DO feel uncomfortable about then he's happy to take a slap on the wrist for that and get away with the rest of it), or he's trying to minimise his own guilt in his head as he knows he's crossed a line. Or both.
What did I learn from my cheating exh? That I was too passive, I never found my anger (well, I did, but I didn't use it in the right way) and I wish wish wish I'd contacted the OW (one of many, actually) and also any of their partners. I wish I'd kicked him out about fifteen years before I did. And I WISH I'd not believed his bullshit when my gut was screaming at me he was lying. That was the main thing... the lying. It stripped me of my ability to make the right choices for me, because any decisions I made weren't based on the truth. Thinking of you, OP.