As you are already aware, the question "Is HBAC safe?" is a simple question that doesn't have a simple answer. The best I can do for you is give you some factors to consider that might help with your decision process.
To begin with, there is another factor besides interdelivery interval to consider: double vs. single-layer uterine suturing during your prior cesarean surgery. Some, though not all, studies have found higher rates of the scar giving way with single-layer suturing. It has been theorized that the variation may be due to differences in suture material and technique, but it remains true that older VBAC studies done before single-layer suturing became popular almost all reported rates of 0.5% or less. The highest scar rupture rate reported with single-layer suturing was 3%, although, as I have said, some studies report rates no higher than with double layer.
Moving on to whether your baby is at greater risk should the scar give way at home, a symptomatic scar rupture will require a repeat cesarean, but in most cases, you have some time, and the babies are fine. Large studies find that with symptomatic scar rupture, the baby's life is lost 5% of the time (1 in 20). With no use of contraction stimulants, the scar rupture rate should be 0.5% or less (1 in 200). To calculate the likelihood of having a scar rupture that results in losing the baby, multiply the two percentages together, and the result is 0.0025% (1 in 4000). In some of these cases, no promptness in response would have saved the baby, and it wouldn't matter if the woman were in a hospital, even one capable of urgent delivery 24/7. That being said, it is certainly possible that being in the hospital during labor might make a difference.
Whether laboring at a home located near a hospital is equivalent will depend on how hospital staff treat home birth transfers. In a situation where your midwife can call ahead, provide all necessary information, and when you arrive, they will be ready and waiting to take you to the OR, you may be no worse off than you would be had you been there in the first place. It is not unknown, however, for hospital staff to treat transfers as if the woman walked in off the street and to begin an assessment from ground zero, losing precious time, which could make a difference in the outcome. I suggest you find out from your midwife which attitude is likely at your local hospital.
On the other hand, it is also not unknown for hospital-based practitioners to say early on that you can have a VBAC and then as the pregnancy progresses, to become increasingly negative on the idea, and if they can't talk you out of it, to eventually refuse too late in pregnancy for you to find someone else. It is also possible for your care provider to have policies that I call the "Cinderella VBAC": you can have a VBAC IF you don't gain too much weight, and IF the baby doesn't get too big, and IF you go into labor by your due date, and IF you progress steadily and rapidly, and IF . . . you get the idea. You would be wise to find out your potential hospital-based care provider's VBAC rate, labor policies, and if they are favorable, whether partners or back up have a similar philosophy and approach. I can also tell you from the research that a woman having a VBAC labor need not be treated any differently than any other woman with the possible exception of electronic fetal monitoring.
I can also tell you that physiologic care practices such as patience, especially with a woman who had a slow labor before, freedom to move about and change position, eating and drinking as you wish, no IV, etc. increase your chances of vaginal birth. VBAC rates are decreased in hospital settings because few hospitals practice physiologic care for anybody's labor, let alone VBAC labors. If you can get physiologic care from staff who are encouraging of VBAC, I see no reason why you would be at any disadvantage in a hospital, unless you have some emotional issues about being in a hospital left over from your previous delivery.
As for whether the likelihood of scar problems increases with labor strength, I am not aware of any research on that point, although it would seem likely that if there is going to be a problem, it will be more likely to show up later rather than sooner. Certainly, that is not the only factor, though, because the while they are rare, scar ruptures occur in women planning repeat cesarean.
As you can see, there are no easy answers and no guarantees of everything turning out as you would hope no matter what you choose. The best you can do is figure out your best option given your individual situation. I hope I have given you some ways of refining that equation. I would love to hear what you decide to do and how everything goes for you.
-- Henci