You don't need a seat reservation use your travel facilities; you only have to make a reservation if you want to guarantee a seat.
The
RSTL Staff Travel Restrictions document lists the handful of trains on which travel facilities aren't valid. Page 13 lists the only such XC services (there are two each day - they are different depending on the day of the week). So unless you were planning to use one of these services, I'm unsure why you would be unable to use your boxes for free travel?
Unfortunately it is correct that seat reservations are generally no longer released 12 weeks before travel, as was the aim before Covid. They now tend to be released around 6-8 weeks in advance.
You can sign up to
alerts on the CrossCountry website to find out when they release Advances, and thus seat reservations. However, unless there is a major event on (such as Reading Festival, rugby, football etc.) then as long as you get your reservation at least a couple of days in advance, it is extremely unlikely that all seats would already have gone.