Sorry not firing into melee but firing through your own team or another creature. The cover rules.
That's capped at -2. There's no provision for 3/4 cover being available here. You can do it, but that's you making a house rule for this situation. Totes fine, just be clear if you're doing this.
If DMs not using the aim rule smart rogue melees switching to range if need be.
Ranged is a better option. You can hide more effectively, your shots are more tactically effective due to freedom of targeting, and you retain mobility without needing to use disengage.
The rogue has hit points use them. You can reduce the damage and you have hit dice as well.
Or you can kill the thing low on hitpoints that you aren't next to, thereby saving party hitpoints even more efficiently.
You're basically screwing your own party over by hanging back unless you have plenty of meat shields maybe.
The idea that not getting hit screws over the party is strange. I mean, I see the shape of this -- x damage handed out evenly to y targets may mean that someone doesn't go down, which may happen if x damage is handed out to y-1 opponents, but this is highly white room analysis that requires a weird form of ceteris paribus to work. I say weird because it means that there's zero adjustment to a different party arrangement allowed -- it's just "team monster hits 6 times and deals x damage per hit. Team heroes has 3x damage. Therefore, team hero MUST make sure to distribute team monster damage to at least 3 characters to prevent 1 character going down and make most efficient use of hit dice (the only resource of relevance in this scheme). I don't buy it.
By all means use ranged if you have to or are low on hit points.
The advantages of ranged:
1) mobility -- you are usually not threatened and so can usually make full use of movement for better tactical positioning. If you do become threatened, then you're tied with melee. Melee never excels here.
2) targeting -- with ranged, you can always make optimal targeting decisions for your attacks as you are not limited to those targets in reach or that can be brought into reach via movement. As such, you can target new additions to the field that benefit, you can target concentrating spell casters, you can target injured monsters to reduce team monster damage output more efficiently. Melee can only target in reach, and 1) reduces the ability for melee to be efficient in targeting.
3) survivability -- rogues make poor frontline combatants. You mention only 1 point of AC, but this ignores things like shields and how a rogue selecting for magic armor means they are not selecting other items that are highly useful to rogues. Once a fighter hits full plate and shield, the AC difference for rogues is going to be 2 points, not 1. Add magic, just +1 armor and shield, and the rogue falls further behind. Also, requiring rogues to be melee primarily means they have to add CON as a secondary pick, bumping INT and CHA to tertiary or worse. This removes effectiveness in the rogue's primary shtick and area of expertise (heh, pun) just to increase staying power in melee. Without CON priorities, rogues lack the hitpoints to remain on the line and effectively absorb hits. Dodge is only partially useful against multiattackers, and removes the one thing a melee rogue really has an advantage on -- the opportunity for OA sneak attacks.
I find your analysis very centered on "everyone must take damage," which is not a strong argument. Or are you similarly saying that wizards, sorcerers, and warlocks should be focusing on touch range spells so that they can effectively soak damage in the line as well?