Huh? Of course they can. You open by weakening, granting you a 15% bonus damage and use the mana regen and flashpoint procs to spam Cages and Immolate (each proc of immolate gets Cage's bonus so with multiple Cages you're doing absurd damage) and detonate paralyzed enemies with Dispel. You don't have to use sleep at all. And if you do, losing the 15% for one second while blasting off a detonator then reapply it with Veil Strike, which is mana cheap, is worth it. Particularly if that detonator is Barrage when Flashpoint is off cooldown because you're then guaranteed to start the cycle over.
And why would you bring Solas if you're playing a Rift Mage yourself? As for Cass? I just hit level 22 on NM last night after killing the last dragon. From the start my party has been Cass, Dorian and Sera. You can bring Cass, you just don't take the passive that causes weakness because, at the moment, the bug can kick in.
Sounds to me that you're messing up the rotation. Both myself and Dorian use crafted Lightning staves with no issue. Open with Pull followed by upgraded Cage, your mana will stay constantly full even if one or two individuals are effected by Sleep. Don't ever use Stonefist unless you intend to sleep the whole group and will kill them with the next attack or you have a Flashpoint proc setup that's going to continue the rotation with back to back Stonefist or Stonefist>Barrage>Veil Strike. It's not hard to keep your passives up near 100% if you're careful and for the short duration they're down you're dealing massive detonator damage anyway.
I don't have Static Cage on my MC yet - usually supplied by Dorian. It's entirely possible I am messing up the rotation. I usually open with Pull and Fire Mine, but then don't seem to have much else to do after. Energy Barrage puts Shocked on the target, Stonefist messes up Weakened, and Immolate overwrites Fire Mine, so has to be delayed several seconds. I usually end up with Energy Barrage and then autoattacking. Maybe I need to dump Fire Mine for Cage, though in a level or two it won't matter as I will have both (only level 14). I'm also missing the Veilstrike upgrade, which would probably help.
How does back to back Stonefists continue the rotation? I've definitely had a Flashpoint proc and used Stonefist twice, but usually what happens is the enemy gets put to Sleep the first time, Ruptured and put back to Sleep on the second one, and I'm now at or near 0 mana. I'm not entirely sure Rupture does much damage, either. Veilstrike only would help if the enemy didn't get woken up in the meantime, which only seems to happen when I ditch the rest of the party. Maybe in a mass AE you'd be guaranteed to get a few of them back weakend with Veilstrike, but then how do you pick out that enemy to attack at combat speed?
Also, what about single target fights? Example: Western Approach Hyena. They are weak to fire, so Pull->Fire Mine.... but then what? They take about 15 seconds for my party to take down at this level (crafted Tier 2 schematic-Tier 2 material gear, except for the one Tier 3 material staff), and Weakened from Pull won't last that long (especially given that 3 seconds of it burn off before the Fire Mine initiates combat). Just about anything I have except Immolate and Fade Step will make it impossible to reapply Weakened. Of course, longer single-target fights isn't a situation Mages are focused at, but it at least feels like KE is more capable of contributing, even if that contribution is boring Spirit Blade spam.
I'm also not sure I could pass up the Will of the Maker passive on Cassandra. I like the mass Sleep/Nightmare mayhem, with no downside on bosses (unless you can't stand tons of Immune spam). I just don't like that it doesn't play well with the Rift Mage's passives.
Also, any advice for actually noticing Flashpoint procs? Is it possible to move the "procs" effect somewhere near the center of the screen so it doesn't require you to look down mid-frenetic-combat?