The thing with this solution is that It's not going to work in real time so you will have to save P.M page manually everytime, Instead (If you could do it), I recommend making it asking for our logs but you should definitely make this open source (GitHub) otherwise people won't use it.
Thank you for recommendation! I will upload it to GitHub or Bitbucket once I have the time.
I agree. It automates the part where you have to manually look for the PMs whether you sent it. But you don't get to automate everything like when there is a new message on your thread, it automatically informs you via the app that there was a new message on the thread and you need to reply to it via PM. If you need to manually input the PMs from your account to the app then just find the messages you need instead. I think that will cost you the same amount of time.
Thank you for the suggestion! To be honest, I did not really get the last few sentences
It certainly makes your process faster since you do not have to manually compare each PM to the replies under your thread, just ctrl + a, save page. However, a way to make it even better is to have a real-time auto-notification when new reply arrives and maybe auto-prepare a PM for that user. Probably, it would not be a good idea to automate everything, though. Because what if someone just asked a question but do not wish to get your PM stuff? So, I agree with most of what you wrote except that "everything" part )) 
