I like where you’re going with 4 - I think that gives a clearer picture of the difference.
surefooted_with_hands_for_balance or surefooted_and_hands_for_balance isn’t much shorter. surefootedness_with_hands_for_balance is perhaps clearer but muddies wording.
surefooted_hands_for_balance makes no sense.
If we can come up with a good terse name for value 4 I’d like to park it pending minor tweaks to wording etc as more feedback rolls in once this gets a bit more formal.
We’re left with value 5, which I think needs to be named after we decide if it’s only YDS 3 / T4 / BMC 1 (simple_scrambling) or if it’s the full range of YDS 3-4 / T4-6 / BMC 1-3 (scrambling). I actually sort of like lopping off T6 and BMC 3 but then we’re left with a verrrry awkward situation where those scales have one value in climbing or we have a very niche odd scramble key.
If we do keep it at YDS 3 / T4 / BMC 1 then I think cramming the higher grades into climbing:grade:=* makes sense, but someone can propose a scramble_scale=* if they are willing to flesh it out.
I sympathize with that being more friendly to what normal folks would be willing to bite off, and agree the top end of scrambling does overlap into easier technical climbing, but then it’s a weird bifurcation of existing systems known and used in the real world. Maybe that’s worth it - people that care about T5 and YDS 4 etc are probably doing their own research / beta and not relying on OSM lol. Those two proposals would have to pass at the same time as they’re dependent on each other.
That’s something I struggle with - if we had hiking hazard/obstacle nodes and it was literally a few mantles here or there over a few miles like some of the trails in Needles NP I’d rather have it be whatever it is the 99.5% of the time and just have the mantles show up on the route in renderers like staircases in Apple Maps and the hazard signs by the trails agency in Hawaii.
I don’t think NPS considers those commonly used trails scrambles, and I never thought I was scrambling, but they do require some upper body strength when you hit those spots. I bet a lot of tourists do those trails that would not be comfortable scrambling. It’s also useful for someone that isn’t physically or psychologically capable of scrambling to see they can take a path for x miles or kilometers (or not) before they hit a mantle.
Barring any other option, we could make a 1m way which is a scramble. 
I originally wanted to toss those mantles in with hands_for_balance (which makes it more use_of_hands), but that’d potentially mess up porting from SAC (it’d be like T3.5) and is also a departure from YDS 2. I do think having a surefooted or attentive trail with a few mantles in it marked up as a scramble is disingenuous to the actual experience.
Side comments on mantles/short use_of_hands
I’ve started writing those up as 2.5 in the Sierra Nevada - mountaineers will consider it 2, many backpackers will complain it was technically 3 heh. Regardless it’s a very different experience from sustained/proper Class 3. Finger Col is considered a Class 2 pass, but has a chockstone on the Blackcap side that needs 2-3 moves to get over. I think letting people know it’s there is useful, but it’s not a “omg scary Class 3 pass climbing with mah heavy pack on” experience. If you can toss your pack on top of it and go up unencumbered that’s a meaningful difference.
While not directly relevant here (I’m really more focused on XC than trails IRL) this is how I fit “2.5” in between 2 and 3.
You are using your hands to pull your body up, but you’re not quite “climbing”. This would be obstacles thigh to head high that can’t be stepped over but you can get a leg over by your third move (and most often sooner). You never have both feet off the ground for more than a few seconds.
Having to butt scoot down something, mantle onto a rock, drop down a short ledge, awkwardly flop a knee or leg on top of something and pull yourself up, etc falls into 2.5. It is the least photogenic class to ascend or descend! Clambering onto and then off large deadfall would be a non-rock example of 2.5. People very sensitive to heights may find it discomforting while those with more experience might not percieve it as any worse than normal Class 2.
Backpacker Note: Travel with a full pack may be awkward or uncomfortable, but Class 2.5 generally won’t cause someone turn back. It is usually possible to lift your pack over the obstacle and go up unencumbered. While even novices are very unlikely to want to be roped in, carrying a length of ~10ft static rope can be useful for lowering your pack down 2.5 terrain (or pull it up after you) so you can go over it unencumbered.
If I was trying to capture trailed terrain I’d also make an intermediary 1.5 ala the WWYDS Class 2 - I actually mentioned that on High Sierra Topix a few weeks ago when I pointed out some of the OSM related threads here in case people were interested. 
In terms of people doing XC peaks and passes I think collapsing casual/attentive/surefooted into YDS 1 is fine - definitely not appropriate for OSM general use!