I see that you are researching this topic. I have found more information about this munition under the rules section for Thermobaric Weapons (IO, p.165). It states "Incendiary LRM munitions are one of the most common types of thermobaric weapons used by the Inner Sphere and Clan forces, but other forms include fuel-air explosives available in both artillery and air-droppable formats". Knowing this, it is hard to believe that there is any damage reduction for using straight Incendiary LRMs. FAMs are some of the most powerful munitions in the game and can even bypass Fire-Resistant Armor's special protection.
For the bonus damage versus infantry, I tried to find the official definition of "volley". It is used for many of the special munitions and for OS launchers (TO, p.327). The best definition I can find is under the construction rules for Missile Launchers (TO, p.326). The rules state "ProtoMechs mounting a permitted missile system may install ammunition by the volley rather than by the ton (to find the per-volley weight, divide 1,000 by the weapon’s listed ammo-per-ton value for a single tube; then multiply that by the number of missiles in the volley to find the kilogram weight of a single volley". This means that volley equals the size of the launcher. This is easy to calculate for normal launchers as you just take the launcher size and divide by 5. But how do you calculate the bonus damage for the irregular sized ProtoMech launchers? Do you round normally, round down, or round up for fraction thereof?