MapShare shows a lot of Activities done early

There is a known problem with activities. They are not subject to date filtering on any of the map pages. This is different from tracks, which do filter by date. Until this is fixed, your only alternative is to either hide all activities (from the Library itself, click the eyeball icon for Activities so that it shows the eyeball with slash) or hide the collections containing the activities so that they do not show on MapShare. You CAN hide the Library from Mapshare, but you must do it from the Filter MapShare Data button on the Social tab for your account at explore.garmin.com.

Your account/MapShare page shows data for all devices registered to that account. If you have only one Garmin account, then yes - it's all the data for all your devices. If you have more than one Garmin account, it's only for the devices on that particular account.

As far as I know, the Mini does not generate activities. It only generates tracks. This is because the Mini is a transitional device - part legacy DeLorme and part current Garmin.

ok, your detailed explanation is very similar to what I have found via my experiments.

Small corrections to your suggestions: hide all activities for Library is working for myself (when I am loged in to https://explore.garmin.com as Garmin user), but didn't work for share.garmin.com/[userMapName]. To hide whole Library is not a solution as all what you want to share will be hidden for visitors.

As result I set followings:

 - created three collections: "iR Active" (set as Active for Plan & Devices - Collections), "iR Storage" - to keep planned routes. These two "iR Active" and "iR Storage" are visible for MapShare (Social - MapShare - Filter MapShare Data (tick checkboxes). Third one created collection is "iR Hidden" to keeps my private waypoint or/and routes, and obviously it is hidden for MapShare. 

 - hide Library for Social - MapShare - Filter MapShare Data (untick checkbox) 

 - all three collections  "iR Active", "iR Storage", "iR Hidden" are flagged to be synced with iR Mini (Plan & Devices - Collections - Synced Collections - tick checkboxes)

Then sync with iR Mini device was performed. Now, I can manage what to keep on device, and what to show for others on share.garmin.com/[userMapName] without unnecessary uncovering too much.

Please comment, is listed above a good solution?

You are doing what I meant to suggest with regard to the library - filter it out of MapShare. 

The rest of it is up to you. The only thing you might need to watch for is collection growth over time. If you continue to add things to a collection (iR Storage, for example), eventually the collection will become too large to sync.