1.) Drum magazines: Yes, reloading drums can be slow. However, in a fire fight session, soldiers rarely have time to reload their magazines, so the loading time is not an issue.
For the US Army, the basic load for a SAW gunner is 600 rounds. A 200 round belt and box is 6.92 lbs, whereas 200 rounds in BetaCo drum magazines is 8.8 lbs, and in 30 round magazines 7.5 lbs. The belt-feed mechanism (top cover) on an M-249 is about 0.75 pounds [from memory], so given the basic load, belt-feed would be lighter. 30-round magazine feed is a close second, though.
2.) Top-loading vs optics: you can angle either the feed or the optics. Angling the magazine well would also help with changing the magazines and reduce the exposed profile of the gun crew during reloading.
3.) Configurable components: ARES's Shrike is a great example of a modular weapon system, with various barrell lengths and belt feed vs magazine feed. However, soldiers generally will leave all of the components behind in the barracks and will deploy with a single configuration.
With the invention of the fully automatic Assault Rifle in World War II, the Light Machine Gun has become much less critical in squad operations. A fire team of 4 full-auto assault rifles can keep up a fire volume similar to that of a fire team with 1 LMG and 3 semi-auto rifles. Therefore, today, infantry squad armament is more a reflection of philosophy than fire volume arithmatics.
There are two schools of thought in infantry squad armament: Generalists vs Specialists. The Specialist school is exemplified by WW2 Stormtroopers and modern American squads: you have riflemen/marksmen, machine gunners, grenadiers, and submachine-gunners in a squad. Each man specializes in a weapon system. In a squad attack, he has a designated role based on his weapon system. He may cross-train on the other weapons, but he is supposed to be an expert of his weapon.
The Generalist school is exemplified by the French All-FAMAS squad. Everyone uses a full-auto-capable, high rate-of-fire, assault rifle. In the Generalist squad attack, the focus is more on the tasks than the weapons: Suppression, indirect fire (rifle grenades), marksmen, and close-in/breach/demolitions. Over time, the squad members end up specializing anyway, but at least their assault rifles help them do everything when necessary.
In an attack, squad/platoon fire (as opposed to maneuver) has two main tasks: suppressing a bunker and cutting off enemy reinforcement/maneuver. A machine gun suppresses the bunker by firing at it as a target. A rifle does so by aiming and firing at the bunker's firing ports/holes. Similarly, the machine gun cuts off maneuver by firing at the group, while the rifle fires at the individual maneuvering. So in the attack, the Specialists and the Generalists engage the same targets; they just go about it differently.
In the defense, the Specialist squad acts different from the Generalist squad. The Specialist squad defense is organized around its LMGs. The LMGs focus on the enemy attack axis, and the rest of the squad focuses on protecting the LMGs. At the same time, the squad needs to site the LMGs away from the other fighting positions, because the LMGs draw fire from the enemy. So the defensive line tend to be a series of one- and two-men foxholes.
The Generalist squad defense also orients on the enemy attack axis. However, it relies on 3- and 4-men fire teams to generate the fire volume (to approximate LMGs). So the defensive line tend to be section trenches (short trenches for the team) or fire team strong points. A fire team strong point can be either one large foxhole or two smaller foxholes right next to each other.
Regardless of the squad armament, both schools have medium machine guns (or general purpose machine guns) at the platoon level. The machine gun may not be critical in the platoon attack, but its value shines through in the platoon defense. Much as its historical role since World War I, the machine gun is a force-economy tool in the defense: A platoon can cover its sector with just the machine guns. It can keep the rest of the soldiers resting and/or under protective cover. At the moment of attack, the machine guns give time for the defenders to get on the firing line.
The reason the Generalist squad can get away without an LMG is because a squad is always part of a platoon defense. A ten-men squad cannot defend a location on its own for long. It can barely keep up an LP/OP while staying on 50% security. Because the platoon MMGs are always available in the defense, the Generalist squad does not need an LMG.
Hope you find it useful in understanding squad operations and machine guns.
Edited to add links and labels.