PEI is one of the best surfaces you will ever find to print on, although I believe one type of filament (I think a variation of PLA?) sticks too well and can damage the PEI trying to take prints off…
That's PETG. I avoid using smooth PEI plates like fire when PETG is loaded. Even after swapping the filament to PLA, little bits of residual PETG can still stick leaving a shadow on the plate. Textured PEI is mostly fine, but single layer stuff like brims are a pain to get off.
From what I understand it tests the minimum retraction distance you need to avoid stringing. The lower you get the less retraction you need. For example, for me usually it stops stringing around 0.4mm retraction (that's 4mm measured from the hot plate), but found that in real conditions the default 0.6mm works better. I don't find this test too useful, for me it fails to demonstrate the spectrum between too little and too much retraction, a feature I appreciate in the pressure advance tower. Apparently the moment it stops stringing, anything after that won't show you anything new and it's best to stop the print. Either that or I fail to notice any defects when the retraction is relatively high.
Lead ain't that dangerous. Just take it out and dispose of it like you do with normal batteries. Clean your hands afterwards and you're dandy. As for the clock, the battery contacts, and whatever they were attached to, are likely eaten away, but I can't say that for certain from this photo. If you're lucky and they're mostly intact, some IPA scrubbing and a dip in vinegar, and a bit more scrubbing, should take most of the crust away. That rust though, probably some vinegar, maybe a deoxidating agent (like navy jelly?) could clean it off. Even cleaning all of it doesn't guarantee that it'll work any way.
I'm thinking about getting some of those activated alumina beads. I've heard they are both more efficient at absorbing moisture and can be recycled indefinitely without degrading. Sounds like a perfect fit for your setup.
There's a freeware (open source but copyrighted art) shooter called World of Padman, that's pretty much what you're describing.
EDIT: You wanted "modern" but, it's slightly less so. Sorry.
PS5 Pro is going for 800 EUR so I've picked these parts: https://pcpartpicker.com/list/BnpTsh, and it's almost spot on how much it costs after converting it into USD. (800 EUR = 881.83 USD)