In addition to @chrillek’s wise comments and questions, re #4 best to read the “free” e-book “Take Control of DEVONthink” available with compliments of DEVONTechnologies, and skim the DEVONthink Handbooks (for both DEVONthink and DEVONthink ToGo). The FAQ would be useful to you now at this stage in your learning. And skim the Blog Articles. All available on DEVONtechnologies’ web site.
Also, do you really need to use a third-party internet sync service? Bonjour method will be more reliable and quicker. Take a look at Sync Types Explained