A vet friend in a very trendy city encounters a lot of cats with significant health problems that stem from their owner's attempt at a vegan diet, so whether or not it's possible, too many people harm the health of their pets through attempting a vegan diet for it to be a safe thing to recommend trying

Intresting paper. It is not the conclusive evidence that you think it is. It's ok, reading science is hard.

Paper concluded that the vegan diet did not seem to have adverse effects, but they had a very small sample size and the expiriment went on for a very short duration.

And then they site scientific papers that disagree with their findings. So there definitely is science out there that disagrees with the vegan diet being ok.

Did you actually read the article? Cause I did and here are some highlights from the article regarding felines specifically:

Sample sizes are tiny

3.2. Feline Studies-Meta-analysis was considered if more than one study presented the same outcome data. However, meta-analyses of these data were not possible due to (1) differences or lack of a comparison group, e.g., a meat-based diet comparator or (2) no presentation of a measure of central tendency or dispersion to input into the model.

Hypokalemia is: a low level of potassium (K+) in the blood serum.[1] Mild low potassium does not typically cause symptoms.[3] Symptoms may include feeling tired, leg cramps, weakness, and constipation.[1] Low potassium also increases the risk of an abnormal heart rhythm, which is often too slow and can cause cardiac arrest

3.2.1. Hematology/Biochemistry-Only three studies [27,29,30] have carried out hematological and/or biochemical analysis of blood in cats that were fed vegetarian diets, and it is worth noting that sample sizes were low. Cats on a high-protein vegetarian diet exhibited hypokalemia which accompanied recurrent polymyopathy [29]. There was also increased creatinine kinase activity, likely reflecting the muscle damage caused by the myopathy, and reduced urinary potassium concentrations.

Myopathy is: a disease of the muscle[1] in which the muscle fibers do not function properly.

3.2.3. Clinical Findings-In cats fed vegetarian diets that were supplemented with potassium, a myopathy was seen within 2 weeks of the dietary change [29]. This was characterized by ventroflexion of the head and the neck. The cats also showed lateral head resting, a stiff gait, muscular weakness, unsteadiness, and the occasional tremor of the head and pinnae....... Weight loss and poor coat condition have also been observed in cats fed vegetarian diets [29,30]. However, most cats in another study had a normal coat condition and no obviously diet-related clinical abnormalities picked up by clinical examination [27]. Clinical signs of lethargy with altered mentation, dysorexia, and muscle wasting, along with gut signs of bloating and increased borborygmi have also been observed [30].

These are guardian based reports which means there is significant bias from the owner to report positive effects and look over the negatives

3.2.4. Guardian-Reported Health Effects-Guardians generally believed that the transition to a meat-free diet had been positive. These studies are valuable, as large sample sizes of respondents (animals) are generally employed. Some guardians did notice an increase in stool volume but noted no issues with consistency [27]. When considering other aspects, coat condition was shinier [27], there was an improved scent of their animals (particularly relating to breath odor) [27], there was a tendency to be at the ideal body condition score rather than being obese [28,31].

This is about as close as you can get to justifying it , IF you fixate on ONE aspect and ignore everything else in the journal article:

Dodd et al. (2021) [31] collected dietary information for 1026 cats, of whom 187 were fed vegan diets. The latter were more frequently reported by guardians to be in very good health. They had more ideal body condition scores and were less likely to suffer from gastrointestinal and hepatic disorders than cats that were fed meat. No health disorders were found to be more likely in cats that were fed vegan diets. The reported differences were statistically significant.

So please explain to me how myopathy setting in and causing tremors after only two weeks of transitioning to a non meat based diet is good for cats?

So for the record you are dead flat wrong by your own damn source because you didn't read it or you ignored all the bad parts.

You are a danger to animals. Stop believing what you want to believe and look at the research