Thing is, he has to take risks. Shana and the Flame Haze are all trying to stop this plan, because it's not an "ideal" solution. Yuji says "Shana won't partake in such a hopeful and risky solution" or something along those lines. That's true. Flame Haze don't act to change at this large of a scale unless there's a guarantee (except a COMPLETELY unexpected outcome like Snake materializing) that it would work. In all honesty, an "ideal" solution won't happen. There won't be a fool-proof way to stop the Tomogara and the Flame Haze conflict. If Yuji can forestall the conflict for thousands, millions of years, so be it. That's better than what the current situation is, and is better for those who are caught in the current predicament. What other solution can someone like Yuji, who is merely only an avatar, pursue in, to try to change the status quo? He's not a God, and he's merely one of the strongest Mystes, which would mean nothing except that he has the Reiji Maigo. Remember, Tanaka said "So Yuji feels as I do, weak, but he's trying to change something". He doesn't think it's the best way - he thinks there's not really another way to solve the problem. He knows he'll probably sacrifice himself for this "imperfect" solution, and he accepts his fate.
I don't think he intends to sacrifice himself for his "imperfect" solution because he did say that the goal of his plan was so he could be with Shana forever. That aside, does the status quo really need changing? It's not like the Flame Hazes were losing or at a major disadvantage. Yeah sure, you can push the conflict back for a really really long time but at the cost of "our world"? That would make a perfect victory for the Tomogara, doesn't it? And assuming some semblance of "our world" survives the destruction, it will still eventually be destroyed because of the imbalance created in Xanadu. Remember that the issue here is the fact that the ways of the Tomogara is creating an imbalance that creates some massive energy imbalance thingy. Correct me if I'm wrong but I think I've got the gist.
Hence, even if they all held hands and moved to Xanadu while singing Kumbaya, if they persist in their actions, their energy imbalance will still affect/destroy our world. So how can that ever be a good outcome compared to status quo? Or what was status quo before he became all chummy with Snake. And, don't forget, the reason why the situation is so dire now is because of Yuji/Snake. So really, was there ever a need to change status quo?
I agree with you on the "he doesn't seek to sacrifice himself". However, forever doesn't literally mean forever. From my understanding of Tomogara and Flame Haze, none of them live forever. Now, a few thousand years or so, sure, that's definitely achievable, but truly "forever" is an exaggeration. That's how long he wants to push it back to forestall the conflict (this isn't really a forestalling, I'll explain this later). That's enough time to spend a "forever" with Shana.
The point about the necessity to change the status quo is because, quite frankly, Yuji's plan is selfish. He even admits it himself in Episode 17. "I can only walk the small, selfish tightrope to the end of this plan, because only an avatar can do this." In the grand scheme of the history of Tomogara vs. Flame Haze, the period of history in which he was with Shana is a mere blip. However, during this time, many many things happened in Misaki City, the Maelstrom of Warfare, and to people he wanted to protect. Yoshida, Tanaka, Satou, Shana, and everyone else around him got badly affected in someway or another, and he wants to avoid this in the future. He also wants to protect Misaki City in its entirety, "forever", because of his whole life being here, wanting to give back to the people of Misaki for what this city has underwent. Even though there hasn't been a spike in violence, so to speak, globally, in this city, with Yuji possessing the wanted Reiji Maigo, there has definitely been a spike in Flame Haze vs. Tomogara encounters, a step up in the amount of strong Tomogara entering the city, though, before this, it was obvious there were still some significant Tomogara plots around the Misaki area - Ex: Shana is fighting Friagne's Rinne at S1 Ep1, and in Shana S the last two episodes reveal that Shana is finishing up some Tomogara in a neighboring city to Misaki and then walking to Misaki to fight the Tomogara (later revealed to be Friagne).
Your point that you're making is that you believe the Tomogara won't renounce their ways of creating massive energy imbalances. That is a Flame Haze viewpoint, and although biased, will indeed cause destruction if nothing changes. However, in Episode 17, Yuji remarks at the history that has transpired between the Snake of the Festival first getting sent to the rift and now. "Tomogara have learned to live with humans. They have gradually learned the need to be with humans, to have humans create and innovate new things for them, rather than taking their power of existence." is a paraphrase of what Yuji said. The ban Yuji made to the Tomogara to not eat humans in Japan is a start to Yuji's plan to possibly, in the future, wean the Tomogara off needing to eat humans and adapt to living with them. Though not liked at first, his orders are still followed, and it is a start. Though Episode 17 begins with what, "Even if he tells them not to eat humans, there's still trouble for us" (whatever female Flame Haze is with Shana at the airport scene), that is due to unresolved resentment between the Flame Haze and their pride and the Tomogara. The FLAME HAZE are attacking, creating the terrorist attacks, on the Tomogara, because their pride, and the reason they made contracts, depends on hating and killing Tomogara (usually, there's a few exceptions like Shana who made the pact willingly).
Similarly, you can argue that the Tomogara resent the Flame Haze (obviously), because of their brethren that got killed by the Flame Haze in the past. Yuji, at least, is trying to change that. The Flame Haze don't seem to see the need to change their actions - partially because their actions are fueled by blind anger and rage, which is why Snake's proclamation was so effective at rendering like 90% of the Flame Haze useless - as with Snake's promise to "end the battle", what point do Flame Haze have in being contracted? In the same line of thought, if Flame Haze decide to stop pursuing and killing Tomogara to "maintain the world's balance" what is the point of their existence? To maintain their sanity, the leaders of the Flame Haze spread proprogranda that gives them "the reason to fight", because they assert that the Tomogara is doing is only forestalling and masking the current problem - the energy imbalance caused by the "status quo" Tomogara. But if Yuji's trying to change that, possibly stop the need for Tomogara to create such energy imbalances - as he implied in Ep 17- then why stop him? He's trying to make peace, while one side seems eager to go with what he says (Tomogara) while one seems to have "the reason to fight" (Flame Haze).
Essentially, my viewpoint is - the Tomogara are willing/trying to change their ways, under the guidance of Yuji and the Trinity, but the Flame Haze are preventing that, because of their fears of being uselessly contracted AND of the unknown - they can't trust Yuji with the balance of the entire world. You were right, on the "energy imbalance" issue. It's just that, in my opinion, that can be solved, with enough effort - an effort Yuji seems willing to take. He doesn't want to "forestall" the issue - rather he wants to work towards a solution (to a sense, that means forestalling the issue until it can be solved), which is different from simply giving up on the issue and pushing it back a few thousand years.
EDIT: Episode 19 confirms that Yuji believes that the Flame Haze can't forgive the Tomogara, by not giving them a chance to change, and do not welcome them on Earth.