It's not so much the "fighting in a line" thing as "fighting in a group".
Age of sail ships adopted line tactics because their cannon were mostly firing to broadside.
Late 19th and early 20th century ships retained line formations mostly for the same reasons, although turrets provided a bit more flexibility. But pre-radio, ships still needed to sail in close formation as all signals were still visual.
By WW2, battle groups no longer needed to sail in line; radio communications meant command could direct individual ships and looser fighting formations were practical. Aircraft had also shaken things up, so looser formations were better for defense.
In space, unless you go out of your way to impose technology that requires some kind of line formation (i.e. David Drake's Honor Harrington books), it's not really a thing. There's no reason to have ships close enough to each other for them to get in each other's way, and in any case they're assumed to be in constant secure communication with each other, so if one does need to move to the side a bit, that is easy to adjust.
I suppose the question becomes what sort of formation a fleet or a group of ship fights in. Staying close to one another to provide shared defensive fire remains a truism as it has since defensive fire became a thing. Ships outside support of one another are easy pickings to get overwhelmed with saturation fire. A ships evasive maneuvers rely upon it to be somewhere else when the incoming fire comes in, and if you fill all possible space with fire than a ship's evasive capabilities are effectively nullified.
WW2 ships still sailed in a formation, with the main ships at the core and escorts at the edge. So would that still be an issue or not depends on multiple things - none of which the game rules take into account.
In the HH universe (David Weber by the way... David Drake did Hammer's Slammers and Ensign O'Leary series) ships used broadsides for combat since their missile tubes used grav accelerators to impart a greater launch speed. And HH-universe ships could not engage any targets at their top or bottom due to their drive mechanisms. I'm sure the tech design lent itself to explaining WHY broadsides-in-space were a thing in his books, especially since it was supposed to be space version of Hornblower. However to give Weber credit, he did lay out a bible of sorts that he didn't really violate too much till later in the series when missiles all of a sudden became off-bore capable firing, but still evidently retained their grav-accelerations - two things that don't function well together in a newtonian universe. Games and books get to take such liberties that reality cannot.
Space gives you full 3-D options, regardless of the technology. Missiles and beam weapons function the same in naval combat as they do in space - they will fly in a trajectory (ballistic vs. straight line) and try to impact with the enemy. Your options are to do nothing if they are going to miss, dodge if it looks like they might hit, shoot them down short of their targets, lure them away with ECM, or rely upon passive defenses like sand and armor and shrug it off. With full 3-D capabilities you can place your heavier ships in the front, your better armored ones, or your escorts. Enemies may expend fire to destroy your screen, or ignore it and focus on the main units.
Someone like Mahan argued for decisive battles, so I'd suspect he'd ignore the shrimp and go for the main line of battle. One could take a balanced approach and have escorts fight escorts (they would be equivalents), cruisers against cruisers and capital ships against capital ships. Of course that assumes fleets are of roughly equal size and capabilities - which often they are not. This is, of course, a purely academic and theoretical discussion as nobody has any real data or tech to model this from. Game rules often don't incorporate the sillier things like reality into their mechanics.
