[SalesForce] Customer Community Sharing

You're correct that the Customer Community Plus license is very different than the standard Customer Community and Partner Community licenses when it comes to their sharing models. You will still have a certain amount of External Sharing vs Internal Sharing that you can configure, but as you say, you can't use sharing sets in Customer Plus Community. That having been said, Apex Sharing, offers you unlimited sharing capabilities, capabilities that either don't exist or just aren't possible within a standard Customer Community.

First, you can create up to 10 Apex sharing reasons per custom object. You don't need to create "explicit reasons" for each record you share. The logic in your code will be doing much of that for you (remember to write your code to unshare records too). Most of it, as I've viewed it in my experience, was along the lines of "related record", "related to account", "related to contact/user/case/record", etc or however else you'd like to describe it depending on context; assuming you don't want to use the default sharing reasons available to you.

The issue then becomes do you want to do the sharing in real time or asynchronously. If asynchronously, do you want to write to a temporary object from a trigger or do you just flag the record? Then, do you run a batch class or do you or use queueables fired from a trigger?

If the latter, I recommend you look at Dan Appleman's pattern in Ver 3 of his Advanced Apex Programming book (released Sept 2015). I used the pattern recently in a very large community+ apex sharing implementation project where the community itself is still in development. I think it's a good pattern. But, it really comes down to having a robust trigger pattern, error handling, plus record processing that can intelligently process and deal with any issues that are encountered with the asynchronous data.

No this is not possible. When you create a customer community user, the user's role is determined by his/her contact's account owner's role.

This determines the level of access the customer community user has on the contacts of his account and other related records. If you change the account on the contact, then the whole roles, access and sharing or related records need to be revoked from the old account and need to be created in new account which is cumbersome.

I could find this documentation mentioning about the roles.

The available profiles for external users are limited to the Partner Community User profile or profiles cloned from it. Unlike a standard user, the partner user role is automatically assigned based on the account name. The partner user role is a subordinate of the account owner’s role. Therefore, all data for the partner user role rolls up to the partner account owner’s role. If you disable a partner user, the partner user role becomes obsolete, and the data no longer rolls up to the partner account role.