Hi check_ya_slater!
I also have a dog who has something similar to spina bifida (a dermoid sinus in her lower back) and she dribbles 24/7 and has to wear a diaper, and we have dealt with some redness. I do not know the exact reason for your dog's redness but I will talk about what I have noticed with my dog. I do not know what kind of diaper you are using, whether it is a disposable, or a washable diaper that you put pads into. My dog wears a denim diaper cover with a Poise pad in it. She is only 8 lbs so I cut the Poise pad in half or thirds. The diaper is snug enough that it keeps the pad pressed against her body. The Poise pad has some kind of gel inside to lock in moisture and dry-weave cover on top so the part that touches her skin stays dryer. With this system, I can give her 2 diapers a day, so she gets one in the morning when she gets up, and then I change her in the afternoon.
She has got redness on her little girl part (nub) and in my case, I think it was because I did not change her in time. There are times when I am 2 hours late changing her, and being in a diaper with a wet pad is the problem. Her little vulva will get red on the inside as well as outside. So it's not just that I have to be sure to change her, but I need to do it on time and not 2 hours late, as we do not seem to have much of a margin for error timewise.
After she developed redness on her girl part a couple of times, I started putting some ointment on her pad. When I prepare her diaper, I lay the diaper cover flat, and press the Poise pad into it. Then I take a tube of triple antibiotic and squirt about 1/2" into the center of the pad where her girl part is going to be. Then I put the diaper on her. I was doing that for quite a while when somebody pointed out that triple antibiotic (Neosporin) includes bacitracin zinc, and zinc is toxic for dogs if they lick enough of it. After that I quit using it for a while, but the redness problem came back. So I continue to put a little dab of Neosporin on her pad with each change. I think that by the time I take her diaper off that she's worn for hours, it is mostly smeared onto the pad, and not on her, but if you worry about the zinc you could always just wipe her little bottom when you change her. You don't want to put too much Neosporin on the pad because I'm pretty sure it won't be as absorbent if the pad has an oily coating of ointment on the surface, you just want a little dab where it will do the most good. I know people talk about antibiotic resistance and not overusing antibiotics. I do not feel like I am overusing it, because her redness comes back when I don't use it, so I think it is doing some good.
I notice when my dog lies on her side, sometimes her urine will bypass the diaper and I will find a wet spot on the kitchen tile. This means instead of going directly into the pad, it is running across her skin, which could cause redness. Fortunately it does not happen all the time and she has been OK, but the more urine runs directly across the skin, the more likely it is to cause redness. My dog is missing one back leg, so there is a hole in one side of the diaper, and that is part of it. But also, I think when she lies in a certain position, the diaper shifts a little and there is an airgap between the pad and her body. This may be happening with your dog. If she lies in a certain position, it is possible the pad is not staying snug against her body and the urine is traveling down her leg into the areas that are getting red.
I do not know if this will help, but there is a brand of dog diapers that offers a "squatty body" size for bulldogs, so you have one that fits your dog better. Maybe it will keep the pad snug against her better. I have not used them, but the compnay has been in business for years, here is the link:
https://www.seasonals.com/products/squa ... dog-diaper
Other ideas I can think of are, do you need a better pad? The pads that come with dog diapers are often not as absorbent as you might need. The ultimate absorbency human pads may do a better job for her. The Poise I buy for my dog are the highest or second highest absorbency. I think many dog diaper pads are primarily for marking, or submissive urination, or dogs in heat, and not for dogs that are fully incontinent. You can tell how absorbent a pad is by getting a measuring cup and pouring water into the pad to see what it will hold. That way you can choose a really absorbent pad. And run your fingertips across the surface to see if it feels wet or dry on the surface when the pad has absorbed a lot.
One other thing is, my dog sleeps au naturel at night, no diaper. I was told it is better to let the skin get some air, and I believe that. This means you will be doing a lot more laundry, because if she is bare-bottom at night she will need new blankets every night. But this may help her red spots heal up. My dog is a good little groomer and it gives her a chance to bathe in her clean crate with her clean bedding, and she washes all her fur and her bottom, etc., which she cannot do during the day when she is wearing a diaper. I think she kind of likes the freedom of being diaper-free at night.
Oh, one more thing. I always wash my hands before I prepare my dog's diaper. This is because I am going to be pressing the pad into the diaper cover and I don't want to get germs on it. I do not think this matters on the leg area, but it might matter on the vulva area. I am not sure how important that is, because to be honest by the time she plays outside I will often see dust and dirt has sifted in the edges of the diaper onto the pad. (My dog is a unique case, she runs by bouncing on her bottom, so her bottom is always on the floor, grass, dirt, etc. Her diaper is a good fit, but dust can get inside sometimes.)
I don't know if any of this will help. I would love to hear more about your dog. Is her name Ivy? (love it!) What kind of diaper does she wear?