Keto diet is not mainly or just for non-vegetarians but a Keto diet means a low-carb diet. It's not about only eating meat. Also, under Keto people try to stay under 20 g, but there are people who can try to go up or down with the level of the carbs intake. Not just that, plant-based low carbs such as avocados, nuts, oils & other low-carb veggies are encouraged.
You will only gain that weight that you have lost if you start eating back again, in some cases you might gain some extra pounds. Thus, it only comes if one starts eating it again as they were eating that time with no exercise & unhealthy habits!
There is a large proportion of fats included in the Keto diet but doesn't mean that you can eat all forms of fats. One must avoid or try to limit saturated ones & should opt for unsaturated fats that come from nuts, fatty fish, oils, grass-fed red meat, eggs, avocados, etc These are just a few of them to name.
Keto diet is celebrated all across the world & is one of the most trending ongoing fad diet for its accurate results on weight loss & it's influence on the fitness & health industry. Also as observed in the studies Keto is safe & doesn't have any side effects when followed for the short term. Also, if you suffer from any medical condition one must consult their doctor before bringing any variation in their diet.
Not forgetting, one will have a low tolerance for alcohol when following a keto diet. But that doesn't mean that you can't have alcohol or it's not an option but one needs to be very careful when choosing the quantity as both beer & wine are high on carbs. But yes, one can consume or have other available options that are low-carb for example light beers, dry wine/other low-carb liquor.