Fasted lifting isn’t a big deal as long as you pace yourself. In other words you need a bit of break in between sets. Instead of 60-90s between sets when training say 12 reps in a set you take 120-180s rests to let the body get enough energy for the next set.
In ketosis you are able to draw energy from fat at a lower pace than on carbs so if you work out harder than what your ketosis can handle you will go into gluconeogenesis which is where the body produces cortisol to break down protein into carbs as emergency fuel in tough situations.
It’s probably not even an issue for you at the moment as you’re new to the gym but after a while you’ll be able to lift more weights which will take more energy which in turn can out you in that situation.
Other than that little issue, solved by taking longer breaks you need to make sure you get enough protein in your eating window. Getting enough protein in a single meal can be tough so I would advice you to spring for some whey protein you can supplement with inside your eating window.
Meh, typically trainers are subscribing to broscience and conventional dietary guidelines that make no sense. As long as you aren’t doing overly intense workouts and have ample fat stores to make it through the day you should be fine energy wise, but you might feel more hungry.
For optimum results you would be lifting at the end of your fast when you are in deeper ketosis and have more hgh and such, then you reward your body with nutrients so it’s kind of simulating working for food. However, it’s not a big deal to be that optimized if you aren’t competing or anything. Especially if you are just starting out I wouldn’t really worry about it. If you do have energy/hunger issues then I would try to situate the workout at the end of the fast right before the feed.
Edit: I forget not everyone on OMAD follows keto. If you are not also keto you probably want to eat after you workout.