DarkMatter, yeah, when you get to HBM II you may find out how useful she is depending on your build.
Mete, 20k shards is a lot, but that's less than a week's haul for another unique hero, which, depending on how much money you spend or how unlucky you are, may be a really important addition considering LB.
I can beat Q, but farm P because, as unlucky as I have been, my main still can't get Vlad even though I have F2P smurfs that have 3 of him, and I only farm Q at about 85%. So in P, without Vlad, Molt, Pixie, or Warlock, she's a pretty good 5th or 6th (I say 5th because I'd dump GR before her) in an all range HBM setup. My buddy who has every hero (except maybe Harpy), can sit in 1st place in Arena, sits on page 7/8 in might, and farms R uses her for HBM, so she may even be better than I think she is. Yes, Santa is obviously better than Siren in every possible way except for maybe the potential to leave stragglers in HBM. And yes, a Pixie with Slow 5 would be almost as good as Siren in ED710, but I'd only use her over Siren if I needed EXP for her, Siren is still better in my opinion because of attack speed and building destruction which is the name of the game there. It is the same reason that Siren is far better than a GR with Slow crests. He only kills trops and heroes which is completely irrelevant in beating the dungeon. Siren may only hit buildings for half the battle, but that's still more buildings than Pixie or GR hit. As for the dual roles, Siren does HBM and ED710, so I don't have that issue. I suppose if you used her in HBM it would make sense to dungeon her too?
My ED710 lineup changes often based on who needs EXP. When my heroes were all capped at 180, I was using a baby 5/9 PD and Cupid. The lower PD helped offset the extra danage Cupid was adding which, if I used the big one, would always kill the boss long before the buildings meaning the dungeon wasn't a "no look". Currently I use my big PD, Mino, Santa, Siren, & Ice Demon with a decoy SM (which would change to Vlad if I ever get him). Siren actually replaced Ork in my lineup when I bought her, and I've never used Cupid (except with baby PD), so I see no reason why she shouldn't replace one of those in yours. Your lineup sounds like it has WAY too much direct damage to boss and not enough global. Are you killing the boss before you 100% the base?