lI have a HD tv with Apple tv . Can I use the Apple Fitness Plus?

That should not be problem at all. Simply sign in using the same Apple ID. You can verify this if you would like (this is a user to user forum) by contacting Apple directly by calling AppleCare. If you happen to be in the USA that number is 800-275-2273, outside the USA please use the following link to find the number in your part of the world.

AppleCare Contact Info

The subscription is for your account, independent of specific devices. Another subscription is not needed.

The service is for your ‘home country’, and may not work abroad.

Yes, please refer to https://support.apple.com/en-us/HT211924

Apple TV HD, tvOS 10

You will need to update to tvOS 14.3 or later. How to update your Apple TV - Apple Support

I would say that depends on the locations you go to and this may be a question you give to AppleCare.