Given her system of might makes right, the simple solution for her enemies is to challenge her to a duel for control of her clan,
Despite this easy way out, Neither Trundle or Tryn has taken it, this means Sej is strong enough that both consider her unbeatable in a fair one on one fight,
It's not that she's the strongest, it's that she's not obviously so weak that they know the outcome of a straight duel beforehand. Some opponents you know how the fight goes before it starts. Like if Superman and my 5yr old nephew had a fist fight. You're assuming that Sejuani is like Superman here, and no one wants to mess with her because they know they'll lose.
I imagine the reality is more like Muhammad Ali Vs Mike Tyson in their primes. Each would probably think they're stronger, and everyone has an opinion, but it's hard to know without it actually happening.
So I imagine the leaders don't wanna risk losing everything over a throw of the dice. For Tryndamere, letting the war run longer is preferable to even a small risk of losing and leaving his people in the Sejuani's brutal hands. For Lissandra, I imagine she's been playing the game too long to find such a risk alluring. You don't live so long by taking unnecessary chances.
That would require the other Freljordians to agree to her philosophy though. And they don't. Sure, she might be physically strongest, but she couldn't negotiate or maneuver in political circles at all. She can yell about how a duel would decide it all she wants, that doesn't make it any more legitimate a method of succession.
The thing is that you couldn't just skip straight to a duel with her. You would first have to duel all her champions, and work your way up. This is especially true of Tryndamere. She isn't going to let some outside king just waltz into her camp and challenge her. He would simply be executed if he did that. She isn't stupid, she knows there are people she couldn't beat. So she plays the system, and makes it by the time you reach her you are already exhausted. That is how she beat Olaf. He had already fought his way through several men by the time he finally got the chance to fight Sejuani.
nobody is stronger then 
!
i for one agree with your logic :D
Maybe it's because she's riding A GIANT BOAR.
It's part of the reason I hate Sej as a champion. She herself isn't a tank, the pig she rides on is the tank. She talks about personal strength, but 99% of what she does in combat wouldn't be possible without that beast she rides.
I promise you if she got off her gigantic boar, 
would decimate her without breaking a sweat.
The problem is that even if they managed to beat Sejuani in a 1 on 1 duel,it'd be difficult to control her tribe,as they would most likely not accept Tryndameres rule. They probably accept that tryndamere is a great warrior,but won't accept the way he rules.
If we go with logic , 
Lissandra is the strongest . She controls true ice , Ashe and Sejuani weapons are made of true ice , and She is so many centuries old , she knows al the secrets of Freljord , and she is so powerful , 
are nothing but the descandants of her 2 sisters Avarosa and Serylda.
"This is but a battle. I've already won the war." -Lissandra
