Don't rely on the letter equating to anything carte blanche in respect of how you use it in relation to them.
I think the original registration date would now be irrelevant because you weren't the original Registrant. It's effectively considered a new registration from the date of the last transfer of ownership as far as a TM predating it is concerned. It isn't as if you purchased the domain with an active website. However if you use it for something completely unrelated to any Registered Trade Mark and in the future someone applies for a Trade Mark in a category relating to your use, you would have prior useage rights and I don't believe they would be able to prevent you from continuing to use the domain name as you were
unless you ammended your usage to take advantage of their offering/goodwill. Please ask your IP lawyer for clarification, though.