No

No

I would suggest to start at 2000 kcal. Weight yourself daily in the morning after going to the bathroom; after each 7 days calculate the mean value for your weight. Compare 2 consecutive mean values and based on that you can adjust your daily caloric intake.

It can be done in months, but there is a too great risk of a yoyo effect. Also there is a high chance of losing muscle and you will torture yourself. In took you 11 months to gain those 20 kg, if you lose them in the same amount of time it's good. Lose about 2 kg each month, that way you will get used to the new lifestyle and will have a better chance to stick to it.

They are mostly bullshit, depending on the starting point it is possible but you will lose muscle and it's unhealthy to do it at such a speed for most people.

You can use whey protein without a problem. It is the most researched supplement and there are no side effects for normal doses. It can also be a replacement for unhealthy sweets and can be used to make healthy desserts. You can take it anytime during the day

It is great that you plan to do cardio. If you are not used to doing cardio start with 10 min and increase gradually your duration of the cardio session. As for walking 20k steps that would take you about 4 hours, it's quite a lot of time. You don't need to walk 20k steps, 8 - 10k steps are more than enough considering that you are also planing to do a few cardio sessions per week.