okay I will

About 80% of Sejuani's take Hunter's Talisman. I think Talisman is better if you're running Resolve because the bonus HP from the rune path will increase the damage of the Talisman Kindle passive right at level 1. Hunter's Machete is still decent and I'd definitely recommend it if you aren't running Resolve.

Inspiration isn't awful with Magical Footwear and Cosmic Insight, but the main reason I wouldn't recommend it is because you lose a lot of attack speed by sacrificing Precision (and Legend: Alacrity. I don't think the runes in Inspiration are worth losing attack speed for your early clear and for stacking up your Permafrost on enemies.

Iceborn Gauntlet is the only "damage" item that I've seen on Sejuani, but personally I'm not a big fan. She's pretty well suited to setting up her teammates and just surviving and tanking damage. I don't feel she gains much from adding damage items that sacrifice her durability.

Turbo Chemtank isn't a bad item. If you're having trouble engaging for some reason, it could help you force fights against slippery teams. I rarely find that necessary though, and prefer to hit big item spikes like Warmog's Armor, Gargoyle Stoneplate, Adaptive Helm and Randuin's Omen which are more effective at keeping you alive. If you can engage without the help of Righteous Glory, it's usually better to build something else.

It doesn't stack with Enchantment: Cinderhulk.

Yeah I miss the old ultimate, but taking power out of her ult meant they could put more into her other abilities (like the stun on E). 5-man stuns were pretty awesome though. :(

I don't think it's better than Aftershock. Fleet Footwork used in pro play sometimes but in regular solo queue games, Aftershock wins way more. I also feel like Aftershock makes more sense as a Keystone, as Fleet basically just gives you a healthier first clear and then some in-combat movement speed, which isn't as good as actual resists/dmg on a tank imo.

Definitely try it out for yourself if you like the active. I've been experimenting with Inspiration secondary more on junglers and it's pretty good. In pro play people are a lot more organized so a single Stopwatch active can make or break an early game, but in solo queue things are a lot messier. I wouldn't say Stopwatch is necessary but it's viable on nearly every champion in the game right now.

The clearest use case is to drop tower aggro during a dive. If you have stopwatch you can make much more aggressive tower dives. Just make sure it's after 6 minutes (so you can actually use the item) and check if your target has a stopwatch too so you aren't surprised.

Hmm, depends what you're doing wrong. In laning phase, play off of your strongest teammate (or the one with the best gank reception). Regardless of how behind you are, if you can land a Glacial Prison on an enemy near a strong teammate, that's probably useful enough.

I'm guessing you might mean in team fights though. If you find you tend to engage fights and then lose them, you're probably forcing too hard. If you can't get a great stun on a priority target, it might be best to save your cooldowns and wait for an opportunity or for the enemy to engage first.

If you're not engaging and then losing fights, you might have to work on your team fight positioning (or you have to find good engages, hehe). Don't be too tempted to dive the backline. Most of the time, hitting the nearest target to your allies and then Permafrosting them is a good idea. I feel that Zac is more of a dive-tank so maybe that could be a playstyle difference that's giving you trouble.

Sorry it's hard to say more without knowing more specifically what you're struggling with. Sejuani is pretty reliable whenever you have ult, even if you're behind, so I'd focus on not dying and making best use of your ults to start.

Alternatively you could always try to perfect your early game and make few mistakes so you don't fall behind in the first place, haha! Though Sejuani's not the best jungler for reliably carrying the early game.