There are a few things to keep in mind here. The main thing is to keep in mind the high degree of uncertainty that the humans have about the operation of the engine, which is alien technology that they've only used a handful of times, and their consequent desire to employ significant caution.
The first thing to note is that, as I understand it, the Resolute was not pulled through the Rift with the Robinsons. At the end of the first season, we see the rift open for a brief time. The Robinsons' Jupiter gets sucked in, and we don't see the Resolute go through. Then we see the crew of the Resolute standing around looking confused. At the beginning of the next season, we see the same scene in more detail, with someone saying that they are no longer in touch with the Jupiter (implying that it went through the rift, and they did not).
OFFICER: Captain, I've lost contact with the Robinsons.
Lost in Space, "Shipwrecked" (S02E01)
Then Robot and the SAR crash into the ship, fighting. It doesn't seem likely that these two went through the rift as well.
But the essential detail comes later:
MAUREEN: But you don't have an engine anymore.
HASTINGS: The second robot, the one that commandeered our ship and
brought us here, it took it.
Lost in Space, "Scarecrow" (S02E04)
The SAR crashed into the ship, used the engine to transport the Resolute to the robot solar system, and then took it. That the Resolute had also gone through the rift was an assumption made by a character, not an authoritative statement. It was an incorrect assumption. The limitations of character knowledge are important to keep in mind for this answer.
Thus, addressing the points in the question:
First, as previously mentioned, we do not know that ships can follow each other through the Rift because of the Resolute, because that is not what happened. We do know this from the attempt of the robot ship to follow the Jupiter 2 through at the end of the season, though.
But as to the reasoning of the Robinsons and company, they do not know that the Rift will be open for at least 18 minutes. What they are saying, based on their interpretation of Robot saying "danger," is that it could be open that long, and thus they are being cautious.
JOHN: Wait, why would taking us to Alpha Centauri be dangerous? Once
the alien engine's activated, how long does the hole in space open
MAUREEEN: I don't know.
JOHN: Long enough for the robots to follow us through the Rift?
VICTOR?: Maureen, we can't lead those things to the colony.
We don't even know that they're assuming that it could be open for 18 minutes, because we don't know how long it takes a robot to open the rift, or whether they are accounting for the possibility that the robot ships could somehow accelerate to a higher speed.
In fact, at the end of the episode, when the Rift is opened, I count it being open for about 30 seconds, or a minute at most. So there are a lot of possibilities here. The most obvious is that they simply misinterpreted what Robot said, and that the real danger was something else. E.g., perhaps Alpha Centauri has been overwhelmed by robot forces or houses some other threat that Robot knows about, but they don't (to be revealed in the next season). Since the signal that Robot detected wasn't even from Alpha Centauri in the first place, perhaps Robot instead was talking about a danger there.
Perhaps the rift closes quickly, but it leaves a trace that can be followed by the other alien ships with their own engines for some time thereafter. Perhaps the size of the rift depends on the size of the ship, and larger rifts close more slowly. That would easily explain all the points in this episode.
But, returning to the question, without knowing for sure how long it would be open, they couldn't make a plan based on the assumption that it would be open long enough for more than one ship to go through, either. In fact, based on the one data point that they likely had (that the crew of the Resolute had seen the rift made for their Jupiter seven months back close in a matter of seconds), they knew there was a high chance that it would only be open long enough for a single Jupiter.
The rift, as mentioned previously, is not known to be open for 18 minutes, and as I indicated it is clearly shown closing about 30 seconds after opening. That's when when Don says "they made it." Maureen, Don, and John were also clearly trying to make it through the rift, but between the short time it was open and having to deal with the exploding Resolute and the alien robot spaceship, they didn't have time.
They didn't consider the idea of going through the rift and repelling the robots there because their plan was a long shot that only succeeded because the robots were singularly focused on them. Going through the rift means running the risk of the robots attacking the humans on Alpha Centauri instead, which they've already said they won't risk. They're willing to take on an increased risk to their own safety to protect the colonists there. Further, the idea of surrendering the engine, as you suggest, is inconceivable: it represents the only possibility of communication between Earth and Alpha Centauri, and thus the only chance of return for any of the colonists there. It also, according to the captain, represents the future of Earth. To save themselves and leave everyone stranded on Alpha Centauri would be unacceptable.