Nice work and very interesting thread D! And (unlike almost everything else on the forums) it's actually something I know a little bit about. For the last couple of years I've basically been making custom kitchen knife handles, as well as sharpening and restorations, for a living. The below are some observations, rather than diktats. As with anything - there are many ways to skin cats.
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Commercial knives, from what i can tell, including expensive commercial knives, are made to prevent people from being able to break them and return them either soon or years later. The combination that you get with them is a shoulder behind the edge and a concession on hardness so that if someone abuses them, they will usually bend.
For knives, I think geometry trumps hardness, but hardness is still important.
This is certainly true, the geometry of a kitchen knife is the single most important factor that affects its performance.
The 'shoulders' or area immediately behind the edge of a knife does serve in part to make the knife more durable, as kitchen knives tend to get bashed around a bit. But it isn't the only purpose; the grind of the main bevel (as opposed to sharpening edge) has a very important role in how a knife goes through food, and how it releases it. Most kitchen knives, Japanese and otherwise, traditionally have slightly asymmetric, convex grinds that favour either right or left handers, depending on the side of food release.
The grind I think you're describing is something like a true zero bevel, or full Scandi grind, and has the potential to make things very sharp indeed, though the downsides would probably regarded as outweighing the positive. Firstly as you note - it's going to be pretty fragile at the edge, though how much of a problem depends on what you're cutting, how careful you are, and obviously how hard your HT is. The other big problem is food release, and related to that - a true zero bevel will actually wedge more than a slightly convex one that's very thin behind the edge.
Those drawbacks though apply to general purpose knives used on boards, for some specialised things like your paring knife or butcher’s knife that's not hitting bone - zero bevels are fine. Yanagiba kinda have zero bevels.
Here are a couple of my own knives with exceptionally good performance FWIW...
This is a Mazaki Gyuto that I've thinned on stones a bit myself. The grind is now very thin immediately behind the edge, but still slightly convexed on both sides of the bevel. Though moreso on the right (left side in the pic cos the knife is upside down obviously), to favour food release for a right-hander:
This next is the same knife belonging to somebody else who was asking for some advice on thinning it. The grind here still has a RH bias, but is heavily convexed and quite fat behind the edge:
Lastly a different one of mine. This knife is very thin with a very pronounced RH bias in the form of an 'S grind'. The left hand side of the knife is almost but not quite a zero bevel, which you can do on the side of the knife that faces into your body because it doesn't affect release. This kind of highly asymmetric grind also requires different edge sharpening angles on either side, to counteract the 'steering' effect when going through food. 70:30 sharpening people call it.
treated in an open forge, which is a supposed no-no
I'm not a particular expert on the intricacies of this, but assuming you can get and hold a reasonably accurate temperature, then an open forge should be fine I think (?) If ever you fancy trying carbon steel - 1080 and 1084 are both really pretty decent knife steels that have wide tolerances during HT and tempering.
Freehand making a handle on an already sharp knife that its totaled if a belt touches anything ahead of the handle
This is the pretty much main reason I like to mostly make wa handles rather than yo / Western handles - you don't need to do it with the knife attached, and it makes it much easier. I'll do another post below with some more thoughts about styles and how I make them, but I like the look of yours there, the design is a bit different from the 'norm' and I think looks very comfortable.
Lastly, the finishing method that some may find useful.
Again - this is very much a matter of personal preference, but... often people don't like high grit, hard high-polish finishes on kitchen knife handles, because they're slippery. And especially pro chefs spend a lot of time washing their hands and working wet. The traditional Japanese Ho wood handles may be boring af to look at, and fairly lightweight, but in terms of grip feel and use they're actually pretty excellent.
Depending on the wood, and whether it's been stabilized or not, I tend to sand to between 180 and 400. And finish usually with a drying oil (Tung usually or BLO), or sometimes just mineral oil.
As nice as it is to work by hand, I think polishing something by hand would be kind of boring.
Haha, yes. You kinda have to learn to enjoy the meditative aspect, otherwise it'd be a feckin drag! Can make things look quite swish though; this is the knife in the first pic above with a nice kasumi polish on it, can't remember exactly what stone I used to finish but it was probably a Maruoyama Suita.
(Fun tip though - Belgian Blue Whetstone polishes in a way that would put many Japanese stones to shame).
