Regarding the first issue, and to some extent also the second, I assume you are considering only the direct victims, but what about the indirect ones?
Regarding pets there is no doubt that adoption mitigates it, but is not enough, you would have to include in the list not allowing the adopted animal to breed or feeding him a vegan diet even if he is a carnivore; something not all, including some vegan pet owners, would agree with. And the adopted animal would have to be a famine one picked in a canil or appearing at the doorstep and not some that some neighbor had allowed be bred and is now giving for adoption.
Clearly some forms of assistance like feeding abandoned cats in the street are just a way to perpetuate and aggravate the problem.
There are of course a lot of things that can go wrong whether the animal is adopted or not, never knew any apartment male cat that hadn't been castrated, for instance.
Veganism is about what is possible and practicable. Unfortunately, we live in a world where animals are abused and exploited at every turn. If we tried to account for every possible factor, vegans wouldn't be able to live. Veganism is about what we can choose, which is quite a lot! It amazes me how there are vegans in extreme circumstances where there are rations and food shortages, and still choosing not to participate in animal suffering.
Regarding pets, there is some disagreement within the vegan community, but in my view, these principles are most vegan:
1. Adopting an animal from a shelter or sanctuary who needs a home and care and might otherwise be euthanised for lack of funds is absolutely vegan.
2. If your pet is herbivorous, it's pretty straight forward. Feed them the plants they're designed to eat. Rabbits, for instance, thrive on hay and veggies.
3. I haven't researched it myself, but have heard from others who have, that dogs, being omnivorous, can thrive on a well planned plant based diet. Certainly the mainstream dog food is terrible for any dog's health.
4. For obligate carnivores, for whom there isn't solid evidence of thriving on plants alone, it seems that there are three options: feed them a plant based diet, in spite its not being healthy for them; let them loose to hunt their own food; or feed them a species specific diet, which would include dead animals. The first, I don't see as being vegan because veganism includes respect for
all species - not just herbivores. To feed a cat in your care a diet which will harm their health, is not vegan in my opinion. The second option would result in a lot of environmental destruction, as cats kill for pleasure as well as for food. The third, I think, amounts in the least harm. It's looking out for the cat's well being, and preventing that cat from destroying many more animals than she needs to for food. Personally, I don't think I'd choose to adopt a carnivore, but I think it's vegan to do so and to feed them a species appropriate diet. Until there's an option that is both healthy for cats and does not amount in suffering, it seems the best option.
5. I think it's fine to spay/neuter one's pet. It often results in a longer, healthier life, and prevents babies being born when we already have more animals than homes. One could make the argument that it violates that animal's reproductive rights. I think, its being in the animal's best interest means it's not exploitative. We have to consider that these are domestic animals, unable to fend for themselves in the wild, and we have to go with the option which amounts in the least harm. We have a responsibility for the creatures we brought into being.
6. Veganism is a human ideal. Other species might practice cannibalism, abandon (or even kill) their young, rape, or kill each other. We don't impose our morals about those things on other species, and it's no different with veganism.