The knockup on Q is practically single target. The times where you can actually knock up more than one enemy champ are so rare that considering it as an AOE ability is overall less correct than considering it as a single target ability.
Yeah whatever
The point is that if you're not stunning multiple targets with your E you are overall far weaker than old Sej. The trade off is effectively an AOE stun VS a single target stun, so without good use of your E, which from the original post Riot released on her sounds like it's fairly reliant on melee teammates your potential is far lower than pre rework.
Although you might not be able to get the a 5 champion stun permafrost is guaranteed point click stun and you get "tons of damage" if you hit a stunned target. In terms of gap closing, you hit a massive AoE slow, majority of champions have dashes less than 500 units meaning if they're in the center they would have to fully dash to get out. Otherwise if they're stuck between enemies and the further side of the AoE they're gonna still be in the slow zone after the dash. Sejuani dashes in, if you miss Q that sucks but you have W which has range now and applies stacks.
Sejuani R is not very reliable at range unless the enemy is grouped up, it's more reliable if you dash and and hit em. New Sejuani R means landing the ability on a champion isn't too critical. Say the enemy is grouped at an objective, you throw your ult but they get out or whatever, you still have a huge AoE of slow zone. This zone of ~1000 units in diameter means that unless the enemy wanna walk back into the slow they're not gonna be dpsing your team unless they have great range champions.
Current Sej can R>E instantly after the R. There is no way that post rework Sej has anything close to that as an option. This isn't even really arguable, pre rework Sej has better options for stacking E, all she had to do was apply a singular instance of damage from any of her abilities and she applied a mark ready to be detonated.
I still think in terms of accessibility E is equal or better. Landing current Sejuani R is unreliable from range because they can dash out. Managing to hit Sejuani R from range means that you need to gap close and during the time you gapclose you're wasting the stun duration. You are able to get E slow on everyone but if that is the case than the ability isn't too different from new Sejauni R. If you're going melee range to land stun R than you're in the same spot as landing new Sejuani R and stacking up. Sejuani's new ultimate means instead of you landing the R on everyone and getting E slows, you're getting a slow zone off, getting AoE stacks with W and then getting guaranteed point click stuns. Having melee allies means more chances for more stuns but shouldn't decrease the potential of Sejuani getting stuns. W is also a spell meaning she can chain her auto attack and W to get stuns faster.
Let's give her a pretty good scenario then, her R is an 80% slow and her E has 0 CD with a 5 sec internal CD per target. Does this actually make up for her loss of her ult stun? IMO, no it wouldn't. What you need to remember is just how weak slows are in League of Legends. Slows do nothing to impede the most popular champions in League right now, Lee, Ahri, Graves, Lucian, Ez, Kha'zix, none of these champs give any fucks about slows, even if the field is large. I need hard CC to pin them down, and I need it to be instant, if it's not they'll ether bullshit themselves away or bullshit me away.
You seem so adamant that slows are bad regardless of how significant they are. By what you're saying Sejuani's current E is worse than the new one. As mentioned earlier unless you get the stun from melee range than the R is unreliable and not used to full potential because, you have to deal with people being able to dodge it and gap closing wasting the stun duration. Afterwards who cares if you land E (even if it's a 70% slow) because everyone will just dash away from you with their "bullshit". So the range stun from R you get isn't that great, and if you get a melee range stun than the differences from the new Sejuani R aren't that huge because if you're melee range you're in stacking range. Also 80% slow is really significant, it means walking away is not an option and it's practically hard CC because you can only use dashes to get away.
In order to get the same effect as her old ult you need to get into melee range. Also the slow field won't enable melee gapclosers, it will enable the enemies to CC your team as a response, knock you away or simply dash away from the slow field. Slows just aren't useful or impactfull in the current state of League, especially when they're tied to long CDs.
Unless the enemy can effectively keep your team at bay for 4-5s while they decide to stay in the AoE slow and use abilities to CC you than I imagine gap closing will be possible. Most of the champions who have dashes don't have hard CC so it's either one or the other or they have their CC tied to their dash. Once again if you get the ult stun from range you need to spend time gap closing, after wards you might get less than 0.75s of stun left, by then the enemy will be free to CC you and dash away from your E slow because it's just that easy to deal with slows as you said. Current Sejuani is either you hit ult and have higher chance of winning or miss and are near useless. New Sejuani has more options and will be as or more viable than old Sejuani now that she isn't an ult bot.