The demon path can be lawful? Also neutral? Angel can be non-good? Bizarre.
Mechnically speaking, the Angel choices are still mostly Lawful and/or Good, but as with any alignment choice, they move your character into certains axis. Choosing a mythic path doesn't necessarily force you to always use the Mythic based choice to solve your problems too which means it is possible to still do Evil actions when you are doing Angel Mythic Path, and vice versa, do Good things when you are Demon/Lich. Now consistently and always choosing your Mythic Path related choice will more than likely move you to the intended alignment and if you role-play your character seriously everything should probably work. Now the game is only halfway finished so I am not sure if there are consequences if, say, initially LN Angel (so you can take the Mythic Path) fall to Evil alignment later and whether the game will acknowledge it or not.
Narratively, usually picking the Mythic-Based choice will result in consequences unique for that Mythic Path but there are also neutral/non-mythic choices in most major decision point. I personally haven't done trial and error to find and compare tho.
Speaking of design, this "range" of alignment is needed to ensure that the player can move forward in the story without being locked out from Mythic Progression all0together. The choice of mythic after all is tied to mechanical action (picking them during level up) and having too strict of alignment restriction might result in a fail-state of some sort, namely can't progress in Mythic Level beyond Level 2 (mythic level 1 - 2 are neutral Mythic Hero path, altho you do get a flavor of choice on Level 1 it doesn't lock you out of anything). This is why Angel and Demon, both are default Mythic Paths that will always be available, have the requirement of only non-Evil/non-Good. You literally can't lock yourself out of Mythic progression.
Another design decision which probably influence this is probably to reduce the amount of complaints. Even during the KS there is no end of people asking question about being a "Good Lich" nevermind the implication of what Necromancy do is Evil in Pathfinder universe. By having a range of alignmet you allow certain degree of flexibility.
Narrative design wise, I think this is realted to the theme of each path as well, altho not always the case. For example, from what few Demon content is in the game (no Demon content in Ch 3 which where all the Mythic stuff happens), you usually has choice of "unleashing rage" or trying to suppress it with your willpower. This lead me to believe that internal route of Demon will be between being consumed by rage as per your demonic nature or trying to keep in check and use your evil power as needed to defeat your adversaries (anti-hero). This way I feel this is logical to have a Neutral or even Lawful Demon as strong discipline is needed to suppress your evil side. But this of course is pure speculation on my part.
