This is a frustrating situation to be in. You obviously care enough about the project/product and/or company to be posting this here which is great. The weapon you need to win this battle is one of Influence.
It sounds as if the senior engineer is functioning as the product owner either by delegation or commandeering. You need to figure out which it is because that greatly impacts your options. Who started the project? Is there a spec that came from outside your group? What other teams are involved? If there are folks outside the team involved, you'll need them as allies to help sway decisions and prioritize. Think about previous interactions with people on this project. Start paying attention to who is talking and making decisions in meetings. Who are people in meetings looking at when questions arise? Who is everyone waiting for in email threads to reply? Figure out who the main players are and make a list, but don't engage them just yet. If this was entirely started within your team, or by the senior engineer in question, then you're going to have to directly work with the senior engineer and your manager to influence change yourself.
If it were me, the first thing I'd do is talk to the senior engineer one on one. You're an adult, they're an adult (presumably). Have an honest and professional discussion. Don't be aggressive or defensive, just talk to them and see if you can understand their motivations behind certain features/prioritization. You might gain new perspective and insight you didn't have before. Or you may inspire them to change their mind. They may also get upset and become agitated. If they have a history of doing this, let your past experiences dictate how much effort you put in here before jumping to...
Speak with your manager. Come prepared with concrete examples to back up your claims, but do not throw this senior engineer under the bus or trash talk them (e.g. do not say things "Soandso is ruining this project by prioritizing useless features."). If there's a spec that was started outside your team, you should read it thoroughly before going to your manager and make double sure that your concerns are valid. You want a civilized discussion that accomplishes several things:
- I may be reading into it too much, but I wanted to put this as item zero. You mentioned "human safety" in your post. If you genuinely believe that there are truly lives at risk here, you need to make this known as loudly as possible. You don't want this on your hands. If you're manager won't listen, stop here and go to their manager ASAP.
- Your manager becomes aware that there is disagreement on the team (if they aren't already). Make it clear that you are looking for them to take a more active role in this project to help mediate discussions on prioritization, ensure the team is working towards the decision (and not just doing whatever the hell they want), and keeping things on track for delivery. This is your managers job.
- Your concerns about which features are prioritized are heard by your manager. Right or wrong, it should trigger them to probe deeper on all sides. Again, concrete examples. Also, bring solutions and suggestions ("I think we should work on X instead because it has Y impact for customers instead of Z impact."). You can mention your one on one discussion with the senior engineer. If you just complain you aren't doing yourself any favors. Ask your manager pointedly what they think of your concerns and how they will follow-up.
- Your manager realizes you are passionate about this particular problem space. This can also help them with your career growth planning and making sure you're enjoying your work.
Ideally, your manager will spring into action and do magical manager things that helps bring clarity and direction to the team. But since that's rarely how things play out... if you've tried a one on one with both the senior engineer and your manager and feel you aren't moving in a positive direction, that's when you get out your list you made above. Schedule more one on one's with these people to get their perspective and to discuss your thoughts. If these stakeholders are reacting positively to what you're saying and in agreement with your vision that's a good sign. If they aren't, you might be in the wrong and it's time to go with the flow.
If you now have several stakeholders who are willing to fight with you, it's time for a big meeting to discuss. Call it a roadmap/planning/alignment meeting, schedule the stakeholders, your manager, the senior engineer, and the rest of your immediate team (don't exclude people who will be working on this project). Prepare for a grueling and painful experience. You may have to do this multiple times. Unless one of the stakeholders step up, you own this meeting, so you are expected to bring an agenda and drive the conversation. Ideally, you've got a stakeholder or two with some authority and they'll help immensely with this part (don't be afraid to ask one you trust to lead the meeting and see what they say). If it were me, I'd start by saying something like "I've spoken with all of you about some concerns I have on certain features/prioritization with this project. I'd like to discuss each one as a group to help drive clarity and direction for all key stakeholders so we can deliver the project on time and to spec." then proceed to go down the list and see where things go. The end result of this meeting should be an artifact that references the decisions made in this meeting and it should be communicated out broadly. Regardless of the outcome, everyone should now be on the same page and hopefully working together. If there are disagreements, you reference the artifact.
Ultimately, it doesn't sound like you are the one that is accountable if this project fails. The senior engineer has either been put in or has put themselves in this position. You'll have to decide if this is a hill worth dying on. Regardless, take this opportunity to establish a better relationship with your manager and the senior engineer to help improve working together in the future. This will not be the last time you experience this.