In original WitP there was no Light Mortars as separate devices, and in WitPAE they are also not present. So your "Grenadier squad" (Knee Mortars) is included in Infantry Squads, this is why they are so large. So, Company consists only from 9 Rifle Squads.
Interestingly, there are also no 20mm AT Rifles present (or Boys AT Rifles), as separate devices, but they are not included in Squads AT power, or their number. That somehow seriously warped USMC, because they get additional firepower from extra support in Battalion, but somehow don't grow in number of men.
Technically, the main difference between HMG, and LMG is tripod (and maybe belt fed ammo). Maybe those MMGs suggests strip-fed MGs? Anyway in-game "HMG Section" consists of two HMG squads, so those are two HMGs, and their crew.
If you want to mess with numbers, remember that every Infantry Device adds 1 AV to unit. So, if you include Light Mortars, or AT Rifles as Devices, they will suddenly add to AV of whole unit.
This squads size reduction is hard to pinpoint, but there are widespread reports of such around 1944. There are reports, that in "Ocean Divisions" squads were divided in half, maybe without additional equipment. Also Shanghai SNLF was reported to have such change, and some units in China. Tarawa Battle Report, on the other hand, describes SNLF defensive positions, as always consisting of TWO mutually supporting LMGs, every of them supported with additional Knee Mortar. That would suggests, that SNLF had TWO LMGs in Squad, and had probably already 7 men separate sections.