Thought at all about going defense and support?

I have, but I'll freely admit the defense tree has never appealed to me much for general purpose. I can see more use for it when you specialize for certain situations, like the arena or specific DT situations for example, but I'm looking to be able to use Scyl and Sethallia everywhere and my impression is that the offense tree is overall better for that. I'm open to changing my mind though, if you have any suggestions.

I would go defense tree for both and get resist at T6.  Getting ACC is easy with percepetion set and GH alone.  For arena and high level DT this will help alot.

So defense and support or defense and offense? I'm pretty keen on that healing boost for Sethallia at least.

Defense and support.  These aren't damage dealers.  Only place I wish I had WM on scyl is to make FW21 a little faster lol.  

Yeah, warmaster is pretty tempting to supplement their damage output against dungeon bosses, but I suppose I should give the defense tree another look. Fearsome presence for Scyl to get her stun rate up is tempting though. Still not sure about the healing vs accuracy route for her either.

Eg  this  was  only  possible due  to  me  going  the  healing  route  for  scyl  healing  on  some  champs  is  far  more  important than acc  black  knoght  died  in  the  2nd  wave  didnt  even  help  much .  This  was  the  last  rotati of  hard  doom  tower  level  50.  So  yes  sometimes  sacrificing acc  for  healing  is  more  importan.

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As  Res  said  above,  healing  is  often  more  important.  When  I  first  started  using  Scyl  I  basically  had  zero  accuracy  and  as  much  speed  as  i  could  squeeze  out,  she  was  just  there  for  the  passive  heals  and  revive.  

And  for  Sethalia,  this  is  my  current  build  but  I'm  debating  dropping  offense  for  defense  

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Well, I've tried putting together a defense/support combo, but having some trouble. Presumably because the offense tree was an afterthought so I had free hands with the support tree, whereas there are things from the defense tree I actually want. Here's the basic templates I've been thinking about, but I'm far from convinced of any of them...

Scyl:

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I'm not sure if the extra stun chance is worth it or if I should aim for the resist instead. If I do go the stun route, maybe it's worth giving up evil eye for harvest despair, but I'm not sure if a 60% chance of a 1 turn leech in the event of a stun that is already fairly unlikely and won't work on bosses anyway is really that big of a deal. Another alternative is to skip both stun chance and resist and get timely intervention or +50 accuracy from the support tree instead.

Sethallia:

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I kinda want that healing route for Sethallia. That 20% team heal (21% or 23% with the bonus) and 15% team TM boost is pretty big, and losing out on it because the target wasn't fully healed hurts. Pushing for that +50 resist hurts a bit, and like Scyl maybe I should give that up and get the +50 accuracy and another T5 support mastery instead of retribution. The accuracy route seems more appropriate for the arena, with a guaranteed 36 accuracy on her initial A3, but a bit less useful for dungeon runs since I don't expect her to ever have no skills on cooldown.

It kinda depends where you're planning to use both, really. For me, Sniper is worth it on Sethallia - at the time (she was my first lego) I used her on my CB team so a higher chance of the HP burn meant more damage, nowadays it's mostly important on DT bosses including Nether Spider and Griffin (the latter she can solo very well if booked, up to a certain point or maybe at any level if you get her sufficiently godlike gear...). Make sure for Sethallia to override her AI because it consistently favours her A3 over her A2, which you often don't want. 

I never even thought of your point with the healing masteries but it's only in a quite small number of cases when the healing target is precisely in that small range of HP percentage that it would work out that way... though admittedly, when I'm playing her manually, I do sometimes heal someone who isn't the closest to death, because that way I'm sure to get the AOE heal / TM boost - or perhaps also because that person needs the block debuffs the most. 

I use Scyl and Sethallia just about everywhere except CB at the moment, but primarily it'll be for dungeons and DT (and FW) I suppose.

Sethallia especially is my primary healer, replacing Apothecary, which makes her heal and TM boost triggering especially important since I lose out on the speed buff from Apothecary. On spider 20, I've often noticed that consistently getting that boost when she heals Drex can make the difference between whether the spider gets a turn or not. Since I build everything for auto, and she targets whoever is hurt the most, I suspect it may take care of some close calls.

For sniper, on the other hand, I've never felt that the 50% (actually 60% since I got bad rolls with my books) chance of HP burn needed to be higher, and 5% is not much if a difference. It's a bigger deal for Scyl since her base chance is 20% and 30% on her A1 and 20% on her A2 unbooked, and I'm hoping to avoid booking her.