The optimum is very much dependent on the materials used (steel wheel, aluminum wheel, single or multiple layers, the kind of spoke nipple used, the spoke thickness, the number of spokes, the shape of the spokes, the length of the spoke and so on). If you don't know exactly what you are doing I would stay away from the optimum and simply play it safe, it doesn't really pay to find that you over estimated one or more of the parts. Overtensioning is much more dangerous than undertensioning, what is far more important is that you tension the spokes evenly, and if you have asymmetric spokes (for instance: most rear wheels) that you tighten the shorter side a little bit more. Consistency is key.
I can't agree. According to the legendary 'Art of wheelbuilding' by Gerd Schraner (and promoted by dtswiss spokes) slack wheels are a recipe for broken spokes. I have heard what you said repeated by many 'old time' mechanics, but the evidence from dtswiss is that all wheels should be tightened to the maximum tension the rim can support. I certainly always did this with my mountain bike wheels when I was a downhill racer and the pro riders I knew agreed. I never had a catastrophic failure and my wheels were maxed out using the method in the book already mentioned
There is a big difference between 'not overtensioned' vs 'slack'. Obviously the latter is going to get you in trouble. But simply don't overdo it, err on the side of caution and don't try to go for maximum tension before things break unless you really know what you are doing. Because they will break if you are not 100% certain that you are still under the limit.
I think the big difference between your use case and mine is simply that I don't have the luxury of rebuilding my wheels every three weeks but they have to last for years. I'm sure that if you treat them as disposables that you can get much closer to the limit, but the bike shop where I worked built ordinary bikes for ordinary people, and then long term reliability is far more important than to squeeze the last little bit of stiffness out of a wheel, though I can see that even in that case there might be some gain from being able to do that.
Keep in mind that this was - and in many places still is - a pretty low tech affair and broken spokes on a bike through normal use on a properly maintained bike in principle should not happen. Usually that indicates that either something got abused, badly mounted or left without maintenance for too long.