Well, I think that all the important facts are nicely covered. With regard to the nasty germs -- and salmonella is only one; campylobacter is another -- the golden rule is that germs increase rapidly in poultry between 40F to 140F. Your sc will surely kill them provided that the birds are defrosted prior to cooking The reason for this is that if you put a frozen bird in the cooker, during the time that it takes for the bird to get from 40F to 140F the bacteria will multiply exponentially. That is often mentioned but not explained. What it means is that if a strain of bacteria doubles every ten minutes, at the end of an hour it will have increased by 64 times (it doubles 6 times, so 2 to the 6th). The incubation time for the little bugs will increase even more if you let the bird defrost on the counter, so let it defrost in the fridge, where it won't get to 40F or on the defrost cycle in the microwave
Are you using the slow cooker so that it will be done when you get home from work? I totally agree that the aromatics that you put in with the bird will have lost their flavor and will also be pretty mushy by the time the birds are cooked, and aside from the time -- about an hour, I'd guess -- that it will take to cook a new batch in the sc. when you take the lid off, it will take about an extra half hour after you put it back on for the temperature to stabilize, so you may want to consider cooking the new set of veggies on the stove top, which will take about 20 minutes.