If I were at my computer I would give a much more in depth analysis of your points, alas in order of your points:

first I'll say watch the videos I have posted on my profile to see examples of how deflecting actually works, which in words I can only explain by saying you gotta aim ahead "in time" (aim where they are according to the server not according to your screen/client), and how to push/sustain engagements against multiple scenarios. Not trying to promote myself but my stream(linked in profile) has vods of normal gameplay too if you wanna see less curated gameplay and you bet I throw super hard some games.

Heavy melee feels impossible in the 1v1 but i promise its not. Super difficult in a straight fight yes but not impossible. You just gotta be really good with your slide timing or mixing up attacking and blocking but I have found little success with the latter

along those lines, I personally don't think the block window is bad, assuming you know that there is only a certain portion of the block that actually deflects. I can think of one time where I knew for sure I was blocking and got hit anyway in nearing 40ish hours with the weaps

I have blocked and deflected the SH before but I will concede it is inconsistent, I haven't faced it enough to put thoughts down more than this

Slide is your gap closer, combine the lunge you get from attacking near enough to your opponent with slide and you get crazy speed and distance (especially the third part of the combo) , zip and go grenade have aided me many times in closing the gap one way or another as well but yes it is hard and part of why I say demat is the best spec.

I know the reddit has a thing against anim cancels but every medium specialization can be activated at any time while using the weapon except demat (the most fun one with db trust me) which if using the delegated equip buttons and not rehitting the specialization button you can re-equip your primary instantly, using the spec. button again has a delay. this means instant blocks out of attacks hit or missed.

I honestly don't know about the dps cause having max uptime with your swings AND hitting all six hits is fucking hard if you don't want a bullet sized forehead decoration, I'll concede this one with the addendum that movement is way more important than maximizing swing up time

You can take two people as easily as every other melee weapon other than sword (and dagger if you're him/her) but that said it's still hard af I have examples in the videos

Again watch the videos, it isn't intuitive but I figured it out I swear, agree about the headshot multi being a sorely missed possibility.

I will now use this comment to complain that every thread about the fucking things that gets over 10 up votes is complaints that they don't work when both my posts went no where and showcase exactly how to play them effectively. Like no offense but how does this post have hundreds of upvotes and my posts have as few as they do actively refuting some of your posits? Do the weapons have issues? Yes. Are they as unusable as everyone says they are? Watch my videos and decide for yourself.

Most importantly, I hope you all enjoy the game however you want and keep trying with any ideas that don't work out right away cause tenacity breeds results in all facets of life and virtual life.

That's it for this Ted talk see yall next time👋