But these changes are design for sej players to either max Q or W first, leaving Sej's E at rank one with 1.5 second 50% slow (10 sec cool down), that is clearly worse than before.
Let's say if you still max E first, if it is even worth it, you are basically spending 50% more mana to do what it used to. That's going to hurt when jungle sejuani's problem is early clearing. But may be you will be fine with a blue buff, I'm not too sure.
Overall, IMO no matter how more often you can spam it, if you can't stick to your enemies, you will have a harder time ganking, burst damage is always better than sustain damage for ganking. When they shift the burst damage to sustain damage and reduces the stickiness of sej, that is going to hurt her a lot. Q and R have a lot longer CD to help you reach the gap again for the second attempt of using E once they have escaped. (Shorter duration of slow and the nerf damage means that you are more likely to do it twice instead of doing it once, without Q or R, I can see it will be harder for sej to even use the E for a second time)
After all is her E still worthy of maxing first?
The main problem is that they might be making her jungling better, but they didn't leave her kit as it is, her E is going to hurt her gank a lot. Even in team fight, you are not that likely to use the E twice if you can't burst down the adc/ap. The reason Sejuani seems mobile because she has her Q and R for her to reach the enemies for her E, but once Q/R are on CD, she will suffer from the chance of even able to use her E again. I understand in late game you have a lot of CDR, but her late game is never the problem.