Attachment size management isn't done through the registry, because it isn't a setting that is stored there. It is within Exchange. Very few of Exchange's settings are stored in the registry.
Therefore to answer your question, no it is not possible to increase size of attachments through the registry. You need to use EMS. As such any change wouldn't show in Process Monitor, so you are wasting your time using that.
There are a number of places that you have to change the limit, at the transport level, at the org level and the mailbox level.
This blog post is the best overview of what needs to be changed:
https://www.codetwo.com/admins-blog/exchange-attachment-size-limit-find-change/