I've read through the whole guide and I must say, good job. I think it's a touch subjective, but you did warn us!
With that in mind, I'm not going to contradict your opinions, but I will try and help you fill in some blanks:
1. I'd remove the mentions of '2-piece combo' etc. This is simply because the number of strikes increases with most combos, up to 5 strikes for most of our main ones at level 80. It's a minor point I know, but I thought I'd mention it.
2. Cleave?
3. Demonic Link - The bonus applies to both physical and magical fire damage, so it *should* apply to all aspects.
4. Spreading Flames - You are correct, the combo affected is Burn to Death.
5. Last but not least, I thought I'd help with the next tier of Avatar (and finish the one you started). Don't feel compelled to use these, I just felt like sharing.
Demonic Leech - Augments the Mind, Body and Spirit Wrack combo by leeching some of the mana burned. Potentially the cornerstone of any Demonic Fortitude builds. (Note, I'm hopeful that they improve the returns on Demonic Fortitude)
Rage of Xotli - Increases the damage of your Hell Strikes combo. The improvement is noticeable, even at lower levels.
Mindless Brute - Increases your hitpoints greatly at the cost of a small but permanent decrease in magical damage. Scales with level and should be integral to any deep-Avatar builds.
Flame of Xotli - Increases the damage of your Burn to Death combo. In the mid-level range I have not noticed any significant DPS increase.
Reaching Flames - Increases radius of PBAoE component of your Pillar of Infernal Flame combo. The radius gain is noticeable and now encompasses large groups.
Demonic Supplication - Grants a new spell that increases the duration of your next Desecrating Essence or Avatar of Xotli spell. This is useful for PvP, but less so for the casual PvE encounter.
Hope I helped in some way.