YZ450F Valves

I don't have the technical answer that you seek --- so wait for others to chime in, as I don't want to mislead you.

A Perspective for Consideration
I used to really stay on top of ALL maintenance - valve adjustments and making sure everything was "perfect" inside the head included. More often than not, my efforts were waaaaay overkill for these modern MX bikes and for "mortal" riders (those not on the rev limiter frequently) - and VERY time consuming. I was honestly making owning a dirt bike more like owning an aircraft (probably like a helicopter - which is very maintenance intensive) ---> more service hours than flying hours (like 4:1). I'm still bad about this, honestly.

Some Maintenance Tricks I've Learned Over The Years --- To Do Less Maintenance :-) 
(1) Change your engine oil often - this helps keep most problems at bay (valvetrain issues included). On my Yamahas, I go 5 hours max between oil changes due to the smaller oil capacity (the small volume of oil gets worked harder than a larger volume of oil doing the same job). I usually need to change the engine oil every 3 hours, however, and the telltale sign for my riding style is the shifting smoothness degrades slightly for me + the oil is no longer honey colored (it's gets darker). My KTMs on the other hand, they carry more oil and seem to work it less - I go 5-10 hours on that oil and it's honey-colored when I change it.
(2) I change engine oil filters every other oil change, sometimes every 3rd --- not with every oil change (I hate to cut open filters to inspect to convince myself this was OK)
(3) I use PC Filter Skins on the air filters (they are like panty hose), as they do an AMAZING job of pre-filtering dirt and preserving the main filter for less maintenance overall. I am not sure why this product isn't talked about more, but I've been using them for ~ 25 years and they have reduced my air filter maintenance by ~ 10X. No kidding.
(4) I gauge how well the valves are doing by how readily the bike starts and runs. If it's starting well/like new, I don't inspect anymore (I only inspect when I do a top-end at like 60-70 hours on a 250F and 125 hours on 450F --- also controversial). For my current type of riding (MX practice rider, trailer rider), I've learned that the valves will tell me if there's developing problems, so I don't need to inspect them frequently. THIS WILL SAVE YOU A LOT OF TIME AND HEADACHE --- AND YOU GET TO RIDE MORE!
(5) More controversy: Maxima Racing oils are my favorite based on personal experience with racing engines in the desert. When rebuilding top-ends and bottom-ends, I compared the same bikes (Honda 450Xs) using different oils under the identical conditions and I saw a marked difference in the Maxima-lubricated engine: way less wear. I also like Motul and Motorex.

Good luck and I hope this was of value.

It's time for new valves/valve job for sure. If you are on the smallest shim and still tight then that valve is worn pretty heavily. It isn't a huge safety factor, most likely it will just become impossible or very hard to start but with a valve that worn there is some risk. I'd be prepared for a new top end, cam chain, valves,seals, valve job

Ideally, yes.  Possibly even new seats installed, depending on how bad they are.    This is basically one of those "do it right the first time" things because you probably don't want to be tearing your engine apart multiple times.