These operations are indeed a tad complex to understand and set up. Zotmoov (and Attanger) were originally written to provide continuity for former Zotfile users when that stopped working in Zotero v6. Those users already understood how things worked - it was just a matter of setting up the equivalent functions in the new plugins. However new users are now wanting to use Zotmoov, and don't have that prior understanding. And not all online resources are correct (as I noted at one of your linked pages).

Re your questions ...
(a) Zotmoov can be set up with a custom menu option to move temporarily-"stored" files back to linked files after mobile use (the opposite of the custom menu option that moves linked files temporarily back into Zotero storage, to make them accessible in the mobile app ... which you may not have found either ?).

Instructions for the Custom Menu are linked from the main Zotmoov docs here:
https://github.com/wileyyugioh/zotmoov/blob/master/docs/SETTINGS_INFO.md

(b) (i) Zotmoov is not involved in file drags into Zotero (AFAIK) - that's default Zotero behaviour ... to make a copy. To get Zotero to add a PDF as a linked file (remaining at its current location) rather than copy it (as a stored file), you hold down a modifier key when dragging, as described under Adding Linked Files here ...
https://www.zotero.org/support/attaching_files#adding_files_via_the_zotero_window

You can also add *single* linked files by downloading them to Zotmoov's Attach New File folder (the associated right-click option then moves the latest-dated file into your "Move To" linked-files folder).

For any *existing* stored file(s) that you want to move as linked files to your "Move To" folder, use right click Zotmoov: Move Selected to Directory.

In order for a file you move into local Zotero storage to be accessible in the mobile app, it must be synced via Zotero servers. So file syncing must be ON (and you must have sufficient space in your online file storage quota). Since linked files don't use Zotero syncing, I guess you could turn it OFF when you're not wanting to get files to the mobile app, but you might as well leave it ON. Note that other files like HTML Snapshots downloaded by the Web Connector will go into local storage (and can fill up your file storage quota if you're not careful), unless you tell Zotmoov otherwise (via Allowed File Extensions).

You can certainly do it just like that.

Setting up Zotmoov's Custom Menus options for those operations just allows you to add some extra operations to those stock menu items, eg to tag linked items that are being moved back into local storage so that they can be accessed in the Zotero mobile apps. That's one way to keep track of them.

You could also set up a collection for keeping track of such items. Drag/copy them into that collection when you are going to move them (via Zotmoov) into local storage/mobile access. And then remove them from the collection when you've finished with them on mobile and have moved them back into linked storage.

To avoid confusion, the particular examples shown in Zotmoov's Custom Menu documentation are for people wanting to do the opposite - they have their PDFs in local storage but want to access them in an *external* PDF reader on mobile devices. That was what Zotfile's old Send/Get From Tablet used to do, in the days before Zotero's internal PDF reader and the Zotero mobile apps. So the PDF is moved into linked storage, in a cloud folder that is mirrored on their mobile device. And then if they wish, annotations embedded in the PDF by the external reader can be extracted into a note afterwards, when the PDF comes back into local Zotero storage.
https://github.com/wileyyugioh/zotmoov/blob/master/docs/CUSTOM_MENUITEM_INFO.md