In a case where you have to face more than one opponent, in a case where putting someone into submission is not enough to end the fight:
If you're more comfortable with the idea of using submissions, then you can train for that: arts/skills such as Chin-Na, some schools of jujitsu, hapkido, aikido... all teach joint locks that let you control one opponent while you're still standing, so you're better able to watch and react to attacks from any other opponents. If they have "friends", you could attempt some negotiation as they hopefully won't want their friend hurt. At a very high level of skill, you may be able to pin multiple opponents.
That said, you have to be massively better than your opponents to use these arts effectively against multiple opponents - and if you're asking about options on here you're at best a decade of hard training away from that. Striking or wrestling is easier to learn and apply.
Does knocking someone out by leading a punch to the chin (or somewhere else on the head) is willing to be lethal ?
Could a brutal blow to the solar plexus leads to death ?
I heard that a solid kick to the groin could also lead to death.
Any strike (and many other forms of attack) might be dangerous... especially when the opponent is untrained, and may have a medical condition. In very general terms, a strike to the head, neck or throat is going to be higher risk to life than a strike to the torso, which is in turn more dangerous than to the legs or arms, but the specifics of the situation tend to be more important than these generalities.
As a point of departure, I'd prefer to kick someone once or twice in the thigh and see if that debilitated them before escalating to other targets. As a kyokushin stylist, I have plenty of experience on both ends of such kicks, and know the average person who doesn't train for them should quickly find their thigh corked and be little further threat unless armed. You can generally do that while defending your head and torso with your arms, and don't have to get as close as you do to punch to the head; there's relatively little risk. And it's much easier than hoping to target specific nerve groups or "pressure points" with strikes or joint locks, the real-world effects of which are famously variable.
But I say "point of departure" pointedly - if you bother to form a mental model of how you'd prefer to manage a fight, many fights won't obligingly fit in. It's good to get used to the unexpected (e.g. by forcing yourself to mix up your strategies/tactics during sparring). As one example - against one charging opponent I charged back with a deliberately gentle gliding side thrusting kick to the chest, the opponent landed pretty heavily and may have hit his head, but not too hard - he could stand up again a few minutes later and walk off.
With the low mawashi geri / roundhouse thigh kick, if you accidentally hit the knee, or if they fall hard or stumble in a dangerous environment - the damage could still be far greater than intended.
All that contrasts with breaking the arm... to be in position to do that usually means you've already taken - or had thrust upon you - many other risks and a longer period of close quarters attack and defence.
Is it ok to break someone's arm (it won't kill him) if you are given the opportunity ? (Through a submission or something.).
Whether it's "ok" is partly between you and your local laws, partly you and your conscience, partly whether you're prepared to deal with all the other consequences. It's good that you're researching and thinking about it beforehand so you're not spending crucial time debating such things with yourself during an actual attack, or under- or over-reacting to a situation in a way you'll come to regret. Once you've got some general ideas about what you think's sensible, talking about it with people whose opinion of you you most value is a good idea too - you want to be comfortable with how they react if you ever have to do something, and it can't hurt for them to know that you are reluctant to injure people and wouldn't do it gratuitously - they'll be more likely to assess any incident from your perspective, then be sympathetic and supportive.
If I had to summarise my impression of the general consensus in the martial arts community, I'd say it was that doing the minimum that's necessary to escape significant physical harm - or protect others from it - is normally "acceptable" to the community, your family, friends and the law, but there are certainly exceptions.
The more skill you have, the lower that minimum usually is.
Whether breaking arms specifically is a better strategy to aim and manoeuvre for than a striking-jaw KO, solar-plexus strike etc. from the perspective of doing "responsible" damage is a very tricky question - arguably needing a statistical analysis in which the specifics would again outweigh the generalities. For me personally I'd never aim for an arm break - though I've train a few years in joint locking arts, I've trained a few decades in striking ones, and it just isn't my go-to skill set or fighting style for serious situations.
All up, if you're in danger and the opportunity presents itself first, and the fight seems desperate enough that you may not get other opportunities, it may be reasonable strategically to take the opening - if you think they intend similar or greater violence. That said, in positions from which you can break, you sometimes have the chance - if you have the skill to maintain control, the time and no other opponents pressing - to let the opponent know that and give them the choice to settle down, or wait for other parties to arrive. That's clearly more responsible than breaking as soon as you can.