If I am reading this right, the purpose you put forth of putting a sewing kit into the first aid kit recipe is to add challenge? You see a problem with how people use the items you give them in-game to heal themselves. So that's what this is about? I am not a doctor either but I stated before I was put through training to be capable of making the right choice if a bleed situation were to occur that needed intervention. My work's response to a bleed situation is something called a "Stop the bleed kit". It usually has sterile packing gauze to pack the wound, and then a tourniquet. It does not involve a sewing needle and thread. If we don't have a kit on hand we make do with something to pack the wound, or slow blood flow to prevent bleeding out.
Nothing about what you are proposing is adding challenge. It instead adds a minor annoyance at best that the players will find a way around. Clearly you want more of a penalty involved when players have a severe wound. Currently if someone breaks a leg, they have to splint it. If they are not careful they can re-injure the limb. You could introduce a similar system for bleeds. Make minor bleeds and major bleeds.Think of it as the difference between getting a surface scratch or cut from regular zombies versus being chomped on by a bear or wolf. One is a more traumatic bleed injury versus the other. Minor bleeds can be bandaged. That doesn't change. Major bleeds need a "Bleed Kit", that involves a First Aid Bandage, Aloe Cream, and a Tourniquet(seriously this would be a nice addition to the game). Once you treat a major bleed the possibility is there to reopen the wound (and bleed again). This forces the player to back off and exit the fight since they have taken a serious injury. This adds an element of challenge because if you take a major injury in fight you can't just spam heal items and keep fighting. This will force you to consider having appropriate armor when picking your fights, and consider the consequences of getting in over your head. Finally make First Aid kits only heal a decent amount of health but not fix major injuries. Remove the Sewing Kit addition, and have them only require what they used to.
I know I'm not a game developer, but I've been playing zombie games for a very long time. I love the idea of added challenge but I'm not sure the current redoing things that already exist is the best way to do that. Especially when they only make things annoying, or don't make a lot of sense.
EDIT: I do truly appreciate that you take the time to respond at all. I am sure you all are very busy and tire of people like me.