i think dracula never meant for jonathan to be alive longer than today's entry. sure, he wanted to keep jonathan around for a few days to toy with him - which is why dracula barely bothered to hide his true nature (the calèche ride and being the driver, the lack of servants, the unnatural strength, not hiding that he doesn't eat or drink, the reverse sleep schedule, the pushing of jonathan's boundaries). jonathan was meant to pick up on all of that, he was meant to realise that dracula was not only a dangerous man, but something supernatural preying on him.
then, after playing with his food for a few days, dracula would have killed him and moved on. he drags out the legal matters until today, but seems to have resolved all of them by now. talking about obtaining the services of multiple other solicitors is both something you would ask to tie up any legal loose ends, and to implicitly threaten your current solicitor - the last veiled threat, before putting everything out in the open.
after finishing this conversation, dracula suddenly stands up and asks about jonathan's correspondence. this is the point where he tells asks jonathan to stay for a whole month longer. i believe, this was dracula purely acting on impulse. he has had so much fun playing with jonathan, he doesn't want to end it here.
it is only now that he provides jonathan with the means to contact the outside world, so that nobody comes looking for him too soon. if dracula had planned on jonathan's stay extending for that long, why not ask him to write back home on the first or second day, when jonathan's suspicions might not have been raised as high and the chance for hidden messages was much lower (dracula is arrogant enough to believe that he would catch any attempt of jonathan's to communicate the situation, but he is also good enough at what he does that he would have minimized any potential risk to himself).
my biggest piece of evidence is that, before going to post the letters, dracula finally gives jonathan the warning to "not by any chance go to sleep in any other part of the castle". this is, of course, because of [redacted for spoilers]. up until this point, however, dracula didn't care if [redacted] happened to jonathan.
today is the day that dracula decided that jonathan is his, and his alone.