Thanks Peter, with my question I was hoping for some cases from a real situation regarding recording actual progress, delays maybe, how do those influence costs, or something else.
Infos and how tos regarding integrations with Apple's apps; Reminders, Calendar and Contacts already exist in the Questions & Answers area: https://www.projectwizards.net/en/support/faq/third-party
"Plan your life with Merlin"... We'll put that to the list to get considered. However it sounds more like a GTD than an application case for Merlin Project.
"Outline of how many projects can be incorporated into 1 Master/Main Project (I see the post from 2018; should this be updated?)"
There is no technical limit. When the file gets too big for your Mac (its memory, hard disk space), you can consider archiving it in a zip and create a new master from the project files which it should include (you may then leave out completed projects).