After doing more playing around with Combat Arts (and catching a good case of wanderlust) I figured out sum mo' thangs, a few that come to mind:
1) East of Teardrop, way in the southeast of the first zone, there's a big area full of T-Energy blue goo, and massive amounts of T-Mutants. I didn't think anything of it, but I wanted to see where the goo went and if there were any quests over there, so I wandered around killing everything that moved, leading to the first point... T-Mutants have a comparatively HUGE chance to drop the mutation sets, which seems intuitive but I never would have known. The only other place I regularly find them is in certain types of chests and other environmental clickies. Given I *did* kill the entire area which was hundreds and hundreds of mutants, I did get almost a full set of Megalcarwen's. They don't really seem to drop much else. Knowing where to find stuff is good information, so it's here. And then holy crap, there's an entirely new area east of the blue goo that I somehow never noticed, it's like finding America in your backyard (in all my playing, story-wise I've never actually gotten past the second area north of the big gate). Also, mutated dragons are pussies... I was wandering, saw a huge dragon appear on my screen and had the self-preservation OHGODITSCOMINGRIGHTFORMETACTICALFLEEING kick in... but once I had cleared an area to fight it, it died in a couple good whacks and didn't even drop anything.
2) I didn't notice until I got my regeneration to stupid-high levels, but I was wrong about Soul Hammer being instant, it most definitely does interrupt normal attacks instead of being an instant blow (at least at 0.2 seconds). I did a fair bit of number watching, and with Soul Hammer on automatic I couldn't find any way to get it to weave with normal blows... the animation completely locks out normal swings and replaces your normal attacks. Given my normal swings were hitting for ~12K and Soul Hammer was hitting for 18-20K, replacing my normal attacks wasn't so bad. Dashing Alacrity alone is 80-110% attack speed at really high levels, and Sword Lore is 40%+ just from Mastery, making attack speed on gear totally pointless with a vanilla cap of 150%. Given I have max attack speed modified to 250% and animation speeds, auto attacks and Soul Hammer were about the same DPS because Soul Hammer was hitting significantly slower, but vanilla games will find entirely replacing normal attacks with Soul Hammer (even at level 1) beneficial. I will have to do some more testing with reading runes and find the level where weaving normal attacks is possible again, will edit when I get around to testing it. Weeellll, off to see a Runemaster about some conversions...
With the game changed back to 150% max speed, using a two-handed lightsaber (I don't think the weapon type matters for 2h weapons, but the handed-ness does, weapon type DOES affect speed for 1h weapons), the exact Soul Hammer timer I needed to weave normal attacks in between was 1.0 seconds (tested on a Bloodclaw boss every 0.1 seconds difference). In Deathblow range, I was doing 24-25k with normal attacks and 48-50k with Soul Hammer every 1.0 seconds... so basically, I was doing slightly more DPS with Soul Hammer at a lower level, doing 39-41K Deathblows but hitting slightly more than twice as fast, and using no normal attacks at all. I assume a good part of that is Soul Hammer's extra damage, Deathblow, extra crit, etc... the difference would be significant on a different build less dumb than my current one. Basically, for quality of life, keep Soul Hammer at level 1 and as fast as possible, or around 1.0 seconds... unless you're maxing it out to use as a massive killing blow, but it takes a LOT of runes read to increase the damage significantly. I didn't test with a 1h weapon, and there's at least two different weapon speeds to test, and I... ugh. Maybe later.
Also, three bosses at once is REALLY rough on only 180K shield, Divine Protection for the photo finish... almost died to wandering werewolves and rats once my shield cancelled itself.
3) Testing at 250% max speed (because that's what I play at, all of combat just *feels* smoother) at 0.6 second Soul Hammer I was able to weave attacks, and killed a Bloodclaw in 16.9, 17.0, and 18.1 seconds. Testing at both 0.2 and 0.5 speed, I had 6 kill times between 21.4 and 23.0 seconds... so there's definitely a point where if you have enough Execution Speed (and Attack Speed to match) weaving normal attacks in between Soul Hammers will give you a significant damage boost. I assume this point exists somewhere for a vanilla 150% speed (probably 1.0 seconds) but the difference wasn't really big enough to bother... it's huge at 250%. I'm all tested out for now, back to doing quests (with my newfound attack strategy).