The 24-week pregnancy ban isn't about the cruise ship not having facilities for pregnant ladies. It's about the cruise ship not having facilities for premature infants. Before 24 weeks, if premature labor happens somehow, or a pregnant woman is injured in such a way that early delivery is indicated, the cruise line can argue that the baby was too small to be viable, and there's nothing anyone could have done, so they were not negligent in not having things to care for preemies. After 24 weeks, they still can't afford the things they'd need to care for preemies (which include considerable staff and special equipment), so they avoid liability by telling women not to cruise at that stage in pregnancy.
As it happens, I had a 32 week preemie, and I find your assertion that preemies at that stage just need warmth and oxygen to be excessively optimistic. My daughter had the benefit of being an anticipated preemie, which means that she had a complete series of betamethasone shots to help improve her lung maturity. Despite this precaution, DD experienced respiratory distress syndrome at birth - her lungs were immature, and wet, and consequently stuck to themselves when she exhaled. This is a fairly common problem for preemies, and in DD's case, it was treated by CPAP (a special, neonate-sized machine that blew air into her lungs), and artificial lung surfactant, which has to be administered via intubation. Do you believe that the cruise ship has these items on board, and has staff trained to safely use them?
You say you'll never be more than two hours from land, but your distance from land is not a measure of how long it would take a helicopter to reach you. A more accurate measure would be how far you are from the closest major hospital with an on-shift helicopter crew that isn't out on some other call, times two, because the helicopter has to come get you, and then bring you back to shore. (Also, the boat can't just go to the nearest port and dock to offload you - cruise ships are huge, and there aren't infinite harbor facilities for them. If the cruise berths are full, or rented to another cruise company, you're stuck on the dratted boat.) If the weather is bad, you should add time to this estimate, possibly days - Nantucket General Hospital admits that, about a third of the time they call for medevac to the mainland, they don't get it.
I think your odds of premature delivery are low if you have no risk factors or warning signs, but you haven't seen an OB, and you don't seem to have a midwife, so it seems entirely possible there might be warning signs present that you have no way of looking at yourself.
I think you're putting yourself and your family in a very awkward position by trying this. The people who do cruise ship check in are not blind or stupid, and plenty of people have tried to get on cruises while very pregnant before you. I believe that the most likely outcome is that medical staff from the cruise turns you away on boarding, and tells your family, very politely, that they can board without you if they want, but you may not board. They will hold firm on this. It is quite likely that some, or even all, of your cruise tickets are not refundable at that point, so the cruise line doesn't care whether you get on the boat or not. If you have not discussed this possibility with your partner in advance, you will have to have that discussion very quickly, and in front of the kids.
I would, personally, be extremely hesitant to tell my children that we were all going on vacation together, get them packed up, and take them to the cruise terminal, if there was a strong chance that I would not, in fact, be going with them. I would be even more reluctant to do all that if I felt that, if I couldn't get on the boat, they wouldn't either. But that's what you're about to do - you're about to pack up your whole family for a wonderful vacation that has about a 95% chance of being a few hours waiting in line and arguing before either you alone, or the whole family, go back home. That is a rotten thing to do to your kids.
Either decide to cancel the whole trip, or decide that you're staying home alone while the rest of the family goes (which might be lovely and restful), and tell the children what the plans are. Don't yank them around by acting like your whole family is getting on a cruise ship for a great vacation.