We can use a window selection and we can also use the TAB key to select several wall intersections at once, according to varying circumstances. If there are no intersections that have already had Disallow Join applied then you can potentially select all the intersections in the same manner as we can select a chain of walls with the TAB key.
It is a bit fussy when using the Allow Join option after using Disallow Join. The walls don’t heal automatically. Initially it seems that using Allow Join only changes one of the two walls at each intersection back to allow a join to occur. The icon for Join appears for one half of the intersection each time. We might infer from this that it is necessary to select a grip and drag or at least wiggle the connection a bit to get them to join back together well.
It is important to realize that there are two walls at each intersection and it is necessary to select them both! If we are careful to select them both then the walls will heal up nicely.
Here's a short video (1 min) to demonstrate it.