You're right about Canonesses.
I use Celestine as a Distraction Carnifex, so I've never gotten her into fights like you've described. That sounds about right though.
I tend to run 5-girl squads with a heavy flamer and plasma priest. I guess it depends on if you're using a CAD or not, as the Heavy Bolter gives good firepower but severely restricts the unit's mobility.
You are correct on Command Squads and Celestians.
Repentia are situationally useful. If you run a ten-girl squad and they successfully get their charge off, they get so many attacks (thanks to Rage) that even if a lot of the squad dies before Initiative, they'll still be able to kill or at least mortally wound whatever they're fighting. I typically use them in this manner against TEQs. They're also good against vehicles thanks to armourbane. All this being said, they're not exactly my first choice for a SoB army due to the reasons you mentioned.
I haven't had the issues you've described with Dominions, but I could see what you're describing being a problem.
Seraphim are primarily designed to skirmish, not kill things. Basically, you find an enemy unit you want to lock down, charge them with Seraphim, and use Hit and Run repeatedly to make sure they can't do anything all game.
You are correct about storm bolters.
Exorcists are pretty much always your best bet. Make sure you're hiding them in cover though because they are really tempting targets.
You are correct about Penitent Engines and Avengers. You also might want to check out Repressors, they're pretty fun.
>Taking three exorcists under 1500 makes people mad.
Yes, but said people aren't usually playing a low-mid tier army like you are, so they can suck it.
>Vindicares are your friend.
So are Imperial Knights (if you have one).
>6++ saves save vehicles constantly. Don't fucking forget them.
Yes, yes, and yes. This is one of the biggest things my opponents hate about SoB.