Good questions. I think you can get used to training any time of day, but it all depends on what you have to do. Obviously work gets in the way for most people and determines when they can hit the iron. Ideally though, I am most comfortable when I have had 3 meals in me. So if I get up at 6:30 AM, I would ideally train around 12 or 1. Most important is my preworkout meal. Everyone digests food at different speeds. Last thing I want is to get hungry in the middle of a long workout. At the same time, you also don't want to be burping up your sweet potato at the squat rack. It can take some experimenting to figure out what a good amount of digestion time is. It can also take some experimenting with food choices. My metabolism is fast, so I do ok with fats in my meal. But some people will only want to use carbs and protein. Pretraining meal can be as simple as 2 sccops whey, 1 cup oats and 2 tbs natural PB. This works well for me at about 1 hour before training. Even better, FOR ME, is beef and sweet potato. Some guys need way more time to process beef. So I'd suggest play around with your timing. But back to the question, you can get adjusted to training at any time. For me, warming up can take longer, earlier in the day. Too late at night, and the physical stress of the day can play a role in how good my workout is. 

As for B12, it all depends on how your body absorbs the vitamin. B12 deficiency is common. As we age, our ability to orally absorb b12 can go down. This is why injections got popular. Years ago, this was the only option, but eventually sublingual tablets came out. 1000mcg injection will work the same as 5000mcg sublingual and will be much cheaper. Tablets are placed under the tongue, where the blood vessels are close to the surface. The vitamin absorbs into the blood stream quickly. This is a water based vitamin. It does not store in your body, so it needs to be taken every day to work. If you don't have a deficiency, taking the pills or injects will do little to nothing. It isn't like a drug. It can't build up like a steroid will. Whatever is extra will just be expelled as waste. If you do have a deficiency, taking the supp may help with energy and even appetite.