Welcome to the forum. Morbe77!
I know for a fact we have at least one other Texan on the forum that manages a vegan lifestyle in the land of sport BBQ. If he can do it, you can too. Hopefully, he'll ring in on this conversation. He might know of some good resources for you to get started.
First let me be clear in saying that I am not a dietitian, doctor, or claim to be an expert, but I do know that obesity can be complex, in that it isn't as simple for some people to just say, "okay, I'm changing my diet". There can definitely be psychological aspects to overeating and/or eating addictions. In these situations professional help is suggested. However, if you are simply overweight like I once was, a whole foods, plant-based lifestyle could possibly be just what you need.
To address your concerns as briefly as possible:
1. Yes, a lot, not all, but many vegans supplement with B12 and sometimes vitamin D3. I know some people supplement in other ways, but if you eat enough of the proper foods with a vegan diet, you should probably only need B12. The truth is that many people, even meat eaters, fall short on B12, which is why I doubt one serving of chicken would meet those needs either. B12 is super cheap to supplement. You can use a sublingual that you can take either daily and some are once a week, and it costs under $20 per year.
2. As I stated above, it's not likely the chicken would meet your needs. The fact is, if you want to reap the health benefits of a plant-based diet, the chicken would be counterproductive.
3. There are health benefits to consuming fruit. Don't you think you'd be a whole lot better off eating fruit than the sugar snacks? Any way you slice it, substituting fruit for sugar snacks will be beneficial. There are fruits that will satiate without the dense calories, like melons for example. I can only speak for myself, but if you follow a whole foods plant-based diet with no sugar, oil, or salt, you should be able to eat as much fruit as you want, in theory.
4. I know a lot of people swear by chronometer. Happy Cow is another app I hear good things about. "Is It Vegan" might be helpful, but I don't use these things myself, so perhaps see what others suggest.
I suggest watching some inspirational and also educational documentaries. YouTube is loaded with great information, recipes, ways to transition to veganism, and also several great documentaries. The best thing you can do is educate yourself on this lifestyle. There is no question it can change your life for the better, in my humble opinion, and feel good for helping the planet, and the animals at the same time. It's really a win win!
A few documentaries I enjoyed on Netflix: Forks Over Knives, What The Health, Cowspiracy
Other recommended documentaries: Eating You Alive, Earthlings (graphic)
Lastly, there's no set rules on how to transition. However, you have to remember there are overweight vegans too. Simply being vegan won't necessarily solve your weight problems. Living a whole foods, plant-based diet will though. There is a difference between these two lifestyles. One is based almost entirely on ethics, and the other on health and scientific research. Look into these plant-based doctors, via Google:
Doctor's T. Colin Campbell, Caldwell Esselstyn, Neal Barnard, John McDougall, Michael Greger, Michael A. Klaper, and Garth Davis. There's a heap more great plant-based doctors, but these ones are a good start. Research them and learn all about the benefits of plant-based living.
This is a way of life for me...Not a diet! While you might be giving up animal products, this is still a complete balanced diet. A huge misconception is the food is bland or boring, or it won't meet your nutritional needs. Again, YouTube has a wealth of information on how to transition, and endless delicious recipes. Get the facts at:
https://nutritionfacts.org/
Best of luck!