What time of day is best when eating only one meal a day?

That is something you need to decide for YOU. There is no best time of day to eat 3 meals or one meal. I have friends that eat one meal a day, between 2pm and 3pm, because it fits with their exercise and work schedules. Another friend eats his one meal after he gets home for the day, which varies by several hours before or after 8pm.

So for YOU, you are going to need to experiment with what works best for you and your needs /schedules and activity levels for a given day.

Well you're gonna get it anyway. Depriving yourself of food throughout the day has an opposite effect on weight loss. You're body will over compensate for the lack of calories and you'll put on weight in the end.

It's actually shown that intermittent SHORT term fasting for a day or 2 once a month is healthy. Try that, and then get multiple meals I per day, but smaller size. No bigger than your fist. You'll feel better and it will work better.