If in fact the valve stem seals and tappets are problematic, those both could be replaced with the cylinder head remaining installed on the engine, assuming you are working carefully with the right equipment. Noisy tappets especially are very easy to replace if you are already doing a timing belt job, just pop the cam out (carefully, using correct sequence on the cam bearing caps), replace 12 lifters, then reassemble with new cam seals. Valve stem seals is much more involved, requiring compressed air to keep the valves closed while springs/keepers/retainers are removed, and a special fixture to compress the valve springs with the head installed on the engine. But certainly doable and a common procedure, everything available for it.
Again, though, serious oil usage from valve guides and valve stem seals is highly unusual on these engines. AND, if the smoke you are seeing is indeed caused by leakage down the valve stems, you would definitely see it after standing 8 hours at work just as much as you would after standing overnight, 8hrs is plenty of time for oil to leak down the stems IF that is what is in fact occurring.
Much more likely, the smoke you're seeing after overnight starts is not smoke from engine lube oil, but rather from incompletely combusted diesel fuel oil in a colder engine after weathering the lower overnight low temp than what the engine sees while sitting at work during daytime temps. That blue fuel smoke would be typically caused by any combination of: 1) incorrect injection/cam timing, 2) one or more dead glow plugs, 3) incorrectly or non-functioning cold start timing advance system, 4) slightly down on compression, or 5) injectors a little tired and spray patterns not 100%. Easiest/likeliest and first things to check would be the timing and the glow system.
Diesel fuel smoke can be blue, white, grey, in addition to the black, depending on the situation and the cause. Blue smoke in a gas engine is surefire sign of engine oil consumption but not necessarily so in a diesel. Blue smoke specifically only upon startup in a gas engine furthermore is a sure sign of valve stem seals/guides in trouble.
So all of your logic is totally correct on a gas engine, but on an old diesel, it's quite different. Especially in the absence of massive oil consumption, I would put valve stem seals way, way down at the bottom of the list for potential causes of the smoke you have seen, after investigating the fuel/glow/timing areas first. Yes, valve guides/stems "possible", but just not a pattern that is often seen.
In general also, a little blue/white haze from fuel for the first minute or two after a morning cold start is quite normal and acceptable.
All that said, if the tappets are noisy, then that IS something you would be for sure right to investigate. We never got hydraulic tappet D24Ts here in the US but the similar relative 4/5 cylinder VW/Audi diesel (and gas) engines with the identical hydro lifters can have the lifters fail and make noise. And in fact just recently I went through this myself when my junker VW TDI with these same lifters experienced a
serious failure that could just as easily happen on a D24 type engine. So yes good to check that out, and if it is definitely tappet noise, make repairs proactively.
Did you try the trick of disabling the vacuum pump to make sure it's not the source of the noise you are hearing, rather than tappet noise? And the trick of killing injectors one by one to make sure it's not a ticking/knocking injector? Note, in a gas engine lifter/tappet ticking is easy to identify since there is nothing else in that part of the engine that can sound that way.... but in the clatter environment of a diesel engine it can be hard to tell the different types of noise apart, and similar noises can come from different components, so organized troubleshooting by process of elimination is time well spent.
Tappets can also be checked individually for excess play using a soft plastic tool depressing them when unloaded. Should only be able to move fractionally and all should sit at the same height out of the head when not depressed.
Yes exactly that would be the method.
Temporary, so the fumes would probably not be too bad to tolerate for a few days, and they are much less stinky in a diesel versus a gasser anyway.
I would put it in place just like you're saying, run the car for a few hundred miles, and see what the accumulated oil quantity is.
But that's only necessary if you think oil consumption rate is high and you're searching for the cause. Even easier, first, would be to just keep track of how much you're actually putting in without making any further changes, as you were already planning. If it's only using a liter between oil changes, then that's not much, and maybe not worth your attention.
All sounds like a good plan, always nice to start with something that you know is squared away on all the basics first and a good platform for upgrades! Plus, you are also right that once you have got a good "Stage 0" fully dialed in, you may find it already much improved and more satisfying even after those steps alone.