Most natural are: Will you be there? Will you be there then? Are you going to be there? Are you going to be there then? If you want to add ‘time’ you should say: Are you going to be there at that time?

First sentence is perfect. Second sentence would be more natural as: Are you going to be home. or Will you be home? We don't need 'at this time' - - you already said what the time was.

The grammar is mostly okay (other than using "this" when "that" would be more appropriate), but it could certainly sound more natural if it was briefer. You're including lots of information that could be understood from the context. I would recommend: Is it okay if I stop by at 6? Will you be home? If you're really concerned that your listener thinks you might mean 6 am (which is unlikely since that's not a common time for social visits), you could include "this evening": Is it okay if I stop by at 6 this evening? If you really want to clarify the time in the second question (which is really unnecessary since you included an exact time in the first question) , I would recommend using "then": Will you be home then? If you really want to use a prepositional phrase "at that time" would work better than "at this time."

That sounds perfectly natural! "At this time" is correct in this situation. "This time" isn't used for a specific hour/minute time but instead refers to this particular moment or interest. For example: Is it OK if we meet at my house this time? (Perhaps last time, you met up at your friend's house) "By this time" is used as a time limit or deadline. For example: Will you be home by 6? (This means that you know they may arrive home much earlier than 6 but as long as they are home by that time, you'll know when to come over.) I hope this helps!