House Rule stipulation: craft incapable of maneuvering are only able to change which segment of a fleet they're in (battle line, reserve) IF that fleet wins the initiative for that combat round.
That way
it's possible to do, but you have to win initiative in order to do it.
Point being that the fleet with the higher initiative is able to "shape" the line of contact in ways that favor themselves ... such as being able to maneuver all the other fleet elements around a craft unable to maneuver so as to "shift" that craft into the reserve "behind" the battle line(s).
However ... if opposing fleets are "unequal" in their "control" of the contact zone "shape" of their line of battle ... the deciding factor ought to be the Roll For Initiative ... no?
Actually, that's wrong (kinda sorta).
If you've got a Near and Far craft, with the Near craft "interposing" itself (blocking) to screen the Far craft ... if the Near craft moves a specific distance (say, 100km for the sake of illustration purposes) then the Far craft is going to need to MOVE MORE DISTANCE in order to remain "behind" the Near Craft.
It's basically a problem of
Parallax.
And if you're dealing with a "3 body problem" of an attacker and 2 defenders, in which ALL THREE are moving ... maintaining the necessary "movement matching" to minimize parallax is going to start getting ... kinda stupid ...

Not impossible ... but moving in the direction of "the juice isn't worth the squeeze" type of effort and coordination.