The level order of the official Alien Swarm campaigns

The answer to your question would appear to be, in fact, yes. I too have looked at the game's source code, most notably in game\shared\swarm\asw_gamerules.cpp and this is what I've found.

There is a number that governs difficulty. In the source it's called m_iMissionDifficulty, though the function GetMissionDifficulty() is often used to read its value. Its initial value is set using the skill level you choose.

• Easy: 3
• Normal: 5
• Hard: 7
• Insane: 10
• Brutal: 13

If there are three marines playing, this number is reduced by one. If there are one or two marines playing, this number is reduced by two. The number can not fall below two (i.e. if you're playing using the easy skill level).

This number's practical application is in various calculations used to determine damage dealt.

This tuning can be toggled using the cvar asw_adjust_difficulty_by_number_of_marines - on by default - though sv_cheats must be enabled in order to disable it.

The easiest way would be to have it indeed as a toggle-able key. In this example I'm using F7

First, you have to go to C:\Program Files (x86)\Steam\steamapps\common\alien swarm\swarm\cfg and create an autoexec.cfg file. If it already exists, open it.

Then add these lines to the file:

sv_cheats 1
alias fps "firstperson;asw_hide_marine 1;asw_controls 0;bind F7 tps"
alias tps "thirdperson;asw_hide_marine 0;asw_controls 1;bind F7 fps"
bind F7 fps

Now you simply have to press F7 to toggle between first person and third person.

By the way, there is no skybox, so the sky will simply be very white, and you can't do hacking in the first person. You'll have to toggle back if you're a Tech.

Note: if the above path doesn't work, try replacing Program Files (x86) with Program Files.