Sounds like you've worked out what you feel comfortable with. ^_^
I thought you said the hospital told you 5-7 minutes, which is why I was saying 5. Sorry for misunderstanding.
Not having your water break is something that happens in fewer than 1 in 80,000 births. You're more likely to have all sorts of strange and fairly rare complications, like the cord prolapsing and causing a major emergency, from getting your water broken by the nurse/doctor/midwife than you are to have it not break until after birth. A baby born in the caul (an intact water bag still over the head/face) is considered very auspicious, a sign of something special. There is also lots of research saying that having them break the bag of waters is not only unhelpful but causes more complications. If you want to know more about birth interventions, complications, possible benefits and risks, I suggest reading Henci Goer's "Thinking Woman's Guide to a Better Birth" and "Ina May's Guide to Childbirth" by Ina May Gaskin. Both are fabulous. I personally think every pregnant woman should read Ina May's book, even if you can't get any other book at all.
A lot of women don't notice when they lose their mucus plug because it comes out while they're on the toilet. It comes out, you wipe, don't really notice anything special.
Yeah, 20+ miles away is a bit far for a carpool. Hah! I was thinking your son might be older than carseat age, so I see where you're coming from with carseats and such.
If you're worried about him being worried about you, maybe you should explain labor to him a little more. After all, your plan is to be laboring with him in the car. Of course, don't share anything you're not comfortable sharing, like the details of things, but explaining how normal it is, explaining what it was like when he was born (in a positive and age-appropriate light), what you're likely to be doing and acting like when you are driving to the hospital, etc. You don't want him to be surprised and scared just because you're acting "different" when he sees you, even if you're not "frantic." I had my younger sister at my first birth, she was in her 20s and had never attended a birth before. She was a little traumatized by how I was acting because it seemed like I was in a lot of pain. It wasn't until I talked to her about it a few months later that I realized at all that she had been scared by the experience, and it wasn't until we talked that she realized I really wasn't in any pain at all until the baby was crowning. We may not realize that others who have no experience with birth can very easily perceive coping techniques in labor as things only done for pain and suffering. If you explain to your son what you might be doing in the car on the drive and that you are doing "hard work," explaining what it feels like without making it sound like you're in tons of pain and having this great hardship, it may help him to not worry about you.
Planning for a car birth is just as important as planning for a long birth or planning for a fast birth in a hospital or any other kind of birth that you're worried about, especially if you feel like you'll be "frantic" if you're in strong labor while on the drive to the hospital or if you have to pull over because you're pushing. Remember that you WILL be laboring in the car. That's not something you'll get out of unless you're planning a c-section or induction. If you want to be less worried about it, think about what would really happen in that situation and how you would cope with it in a non-frantic way. You'd be having a fast labor and birthing in the car because you're healthy and the baby is ready and everything is working the way it's supposed to. What would you want with you? Some clean towels and possibly a suction bulb is all you'd probably really "need" to have with you, as you wouldn't even want to cut the cord yourselves. Talk to your midwife about an emergency birth plan, what to do, who to call, when to pull the car over, etc. Even though it's not likely to happen, you're worried about it and you may very well have two hours or more before you get to the hospital from the time when your husband leaves work, which could be far into early labor or active labor. If you've worked through the scenario in your head and with your husband, you'll feel at least a little better about the possibility because you will have prepared for it. You already know you want to avoid it. But what will you do if it actually happens? You'll have a baby, your husband will catch him/her, your son will witness the birth of his sibling, you'll get to the hospital soon thereafter and they'll all talk about how great it is that everyone is okay. Why? Because birth is normal and normal birth is usually healthy. Only 17% of planned home births end up in the hospital and only 3-4% of planned home births end up as surgeries. So you have a huge chance that everything would be absolutely perfect as far as health is concerned. Remember in your scenario that you would be pulling over to birth the baby. If you're feeling the uncontrollable urge to push, your husband needs to pull over. He needs to NOT speed up or keep driving. Even ambulances will pull over if the mother is uncontrollably pushing on the way to the hospital (I took EMT training, decided it wasn't for me). An ambulance will pull over to deliver a baby, so your husband would need to also do so. You can buy emergency birth kits for $10 at http://www.1cascade.com, if you're really worried about it to the point of wanting full preparation.
It just seems like something you're really worried about. I personally feel like anything a mom (or dad) is really worried about should be addressed, stared in the face and figured out. If you really DO end up with a car birth, it would have helped you tremendously to have worked through the fears, and if you don't end up with a car birth, you still would benefit from the mental preparation because you should end up feeling less stressed and more prepared for a variety of situations.
I don't mean to pester you about it or push you to do anything you don't feel comfortable doing. Your plan sounds just right. I'm just thinking about what I would need to do for my own mental preparation if I were in your situation and feeling the way you seem to feel. When I was pregnant with DS2, I lived 45 minutes from the hospital, my husband worked in the town of the hospital, we had one car, my home birth midwife lived 1.5 hours away, and right when I seemed ready to have the baby, my husband was sent out to a fire (he was a wildland fire fighter). This meant he wasn't likely to get to me for hours and hours, if he got to me at all, if I called him at any point in labor. Talk about stress... I really had to cope with the idea that he might not be there at all, though in a home birth I didn't feel like I would be birthing alone (unless I birthed in less than the 3 hours it might take my midwife to get to me). Still... I know everyone has a scenario that just might happen in their labor and birth that scares them. I was actually more worried with my second birth than with my first (lived further from hospital, husband had job where he was in the mountains away from phone contact all day, midwife lived far away, lived away from family and normal support network, etc). It really helped me to work through what I would actually do and be prepared for it so I wouldn't fall into a panic if it really did happen. That's honestly why I stress mental preparation and working through it to alleviate some of the fear.
Well, good luck! Everything will probably work out great. You'll have plenty of support at home until you're ready to leave, you'll leave at the perfect time and have an awesome birth where you have planned it all along.
Oh, and I had the baby the day after DH came back from the fireline... Guess I was holding back. Heheh.