As I mentioned in the comments, you're looking at the wrong aspect of armor. You shouldn't be looking at the percentage it reduces, but rather the amount of damage it actually prevents as a measure of how much longer that armor lets your survive.
Let's consider an example for a champion with 0 armor. Say he has 1000 health, just 'cause that's a nice round number. Now our champion gets caught in Garen's judgement, and starts taking 100 physical damage each second (at one hit per second). Clearly, our protagonist can take ten hits before dying.
But now, imagine our champion has the 50 armor you mention, meaning the incoming physical damage is reduced. Now (still with 1000 health) it takes 15 seconds for that bastard Garen to claim our champion's life. 50 armor extended our life by 5 seconds. (100 * (2/3) = 66.6 DPS; 1000 / 66.66... = 15 hits)
So our champion respawns, wises up, and buys 50 more armor. Now with 100 armor, we're sitting at a flat 50% damage reduction. Again we run into Garen, who's still dealing 100 base damage per second. It now takes him twice as long to kill us -- 20 seconds. Once again the armor has extended our life, again by 5 seconds. (100 * .5 = 50 DPS; 1000 / 50 = 20 hits)
Back in base, we buy another 50 armor. We're now at 150 armor, and 60% damage reduction. Enter Garen -- spinspinspinspinspinspin and it takes 25 seconds before our champion dies. Again, adding 50 armor extended our life by 5 seconds. (100 * .4 = 40 DPS; 1000 / 40 = 25 hits)
So even while the amount of damage reduced per point of armor diminishes (as you observed), the amount of time armor extends your life (or effective health) remains distinctly linear.
True damage does exactly the listed damage, as True Damage is not reduced by armor or magic resistance, nor by any flat or %reduction granted by champion abilities.
In the past, true damage was subject to the same multiplicative modifiers as magical or physical damage -- i.e., doing more on a Hemoplague'd target, less on Alistar's Unbreakable will, etc., however it appears that Riot considers this a bug, as they have been slowly "fixing" true damage dealing reduced effect to Olaf's Ragnarok, or while under Sona's Diminuendo debuff (for example).
The reason this change was made is because smite used to do magic damage, where it was particularly effective against the jungle creeps (who had negative magic resistance, and thus took extra damage from smite). Recently, the mobs were patched to have non-negative magic resistances, so smite lost effectiveness, even though ability itself wasn't changed.
Best Answer
You can't tell the percentages, all Mixed damage means is that damage came in multiple types from one skill. Sometimes it's a combination of Physical and True (Irelia), Magical and True (Ahri), or Physical and Magical (Fizz).
Liandry's Torment procs off of all activated abilities (Not on-hit effects), whether physical, magical, or True. It will work for abilities like Cho'Gath's Vorpal Spikes, as they trigger a spell activation on attack, rather than an on-hit effect. The same will hold true for Rylai's Crystal Scepter, so this item combination will work very well together.