I have a subscription plan that is only for a specific user or set of users, and I’d like to groom the presentation (and subscription sign-up) of that plan to only the user or users to which it applies. 

First, I want to make sure I’ve set the plan workflows and access so that the plan does not show for other users to which the plan does not apply. 
Under Plan >> Workflows I’ve set the following:
Selected by default >> No
Allow Registration >> No
Allow upgrade to this >> No
Under Plan >> Access I’ve set the following:
Published >> Yes
Allow new subscriptions >> Yes
Allow access from site frontend >> No, this plan is not visible in site, only in admin backend.
Are these the correct settings to ensure this plan is never presented publicly?

Second, intersecting with those setting above is the challenge of presenting the plan to the appropriate user(s) for initial subscription sign-up and subsequent renewal. Is there a way to present a plan sign-up only to a specific user or set of users…maybe available only in their user profile subscriptions tab? Maybe this is something exclusively managed via the Joomla admin backend?

Finally, this is potentially a situation where I’d also like to apply the CBSubs Family Plans plugin. 
Can Family Plans be managed via the Joomla backend or are those groups only manageable via the emails that the Family Plans plugin sends? 
Also, when one person of the Family Plan signs-up or renews, I’m assuming the subscription become active for all the others on the Family Plan, correct? 

krileon wrote:

Are these the correct settings to ensure this plan is never presented publicly?

If you intend for it to be entirely managed from backend then yes your settings are correct as that plan cannot be accessed from frontend using that configuration.

Second, intersecting with those setting above is the challenge of presenting the plan to the appropriate user(s) for initial subscription sign-up and subsequent renewal. Is there a way to present a plan sign-up only to a specific user or set of users…maybe available only in their user profile subscriptions tab? Maybe this is something exclusively managed via the Joomla admin backend?

Given your plan can't be accessed from frontend with that configuration you'd need to entirely manage it from backend in which case you'd just give the plan to the users who are allowed access to it manually and there's no further configuration required.

Finally, this is potentially a situation where I’d also like to apply the CBSubs Family Plans plugin. 
Can Family Plans be managed via the Joomla backend or are those groups only manageable via the emails that the Family Plans plugin sends? 
Also, when one person of the Family Plan signs-up or renews, I’m assuming the subscription become active for all the others on the Family Plan, correct? 

Shared subscriptions can be managed from the backend as well. Navigate to CB Paid Subscriptions > Payments Center and you should find a button called Shared Subscriptions. From there you can create, edit, and deleted shared subscriptions.

Relative to the first two questions, is there any implementation middle-ground, where plan can be presented on the front-end, but only to a certain set of users?