Is it not possible to get the exact recipients using "Main Mailer Triggering Conditions" since you can specify before/after date ranges there directly? Probably something like the following.

Automatically send message starting at: 1 month before
Relative to this date: Subscription's Expiry Date
But do not send message if it's later than: 1 month after

I don't find anything in substitutions tutorial about creating a substitution string representing a future date as an offset from today -
e.g. [cb:date format="PHP_DATE()_FORMAT" date="DATETIME + 30d" /]

Such a substitution doesn't exist. You'll have to use the built in +/- date conditions in the condition dropdown to modify a date.

Ok, so you want it to be 30 days before/after expiration date AND if their expiration date is 30 days before/after the date the mailer was processed? That's doable. Keep your current configuration then add the following condition.

Date A, Field A or Value A: Expiry Date of existing user subscriptions above
Date B, Field B or Value B: Constant Value or String (CB substitutions can be used)
Value B: [cb:date format="Y-m-d H:i:s" /]
Condition 1 (> for Dates): For dates: Date A > Date B - 30 days (Date A must be later than 30 days before Date B )

I believe that should allow any subscription with an expiration date within 30 days of datetime of the mailer processing. I can't be completely sure this will work though.

I don't fully understand why the need for the condition though. You can control whom should be emailed using the before/after parameters.

What config setting can prevent Users A and B from getting into the queue?
or from being sent? - But do not send message if it's later than: 1 day after ?

I don't know. I don't understand what you're trying to accomplish exactly. If it's meant to email them before they expire then don't set an after parameter or yes set it to just 1 day. If it's supposed to be "you're expiring soon!" notification then the below is probably ideal.

Automatically send message starting at: 1 month before
Relative to this date: Subscription's Expiry Date
Must be in state: Active
But do not send message if it's later than: And no other date condition

What happens to emails in the send queue that aren't sent?

Nothing. They just sit there until the sending queue is processed.