Huh... I like that they put thought into a demon-blooded Tiefling and how to make it different from a devil-blooded one, but... random spells?... I get that demons are about chaos, but generally speaking do demons just randomly change into other demons once a day?
Plus, having random spells is never going to be equal to having the same spell because if you know what spell you are getting, then you can build your strategy around it and the DM can present you with opportunities to use it. With a random spell, even if it is the same level the problem is that you are stuck in the situation of looking for a time when you might possibly make any use of it.
Also, I'm not sure about putting +1 Con and bonus hit points onto the same subrace.... But, then again, Dwarf has a subrace that grants twice as many bonus hit points and it defaults as a +2 Con race.
The spells are a nice and flavorful... Not sure I would want PCs using them, but I could easily imagine a villain who might sacrifice innocents (or even her own minions!) to summon demons which could be uses as an interest point for an encounter. If you are already fighting the cultists, you got the choice of either trying to cut through the cultists or trying to get through them, likely incurring opportunity attacks, in order to try to stop the warlock from killing the hostage (and even then you got to choose between doing damage, hoping that it will be enough, or forgo damage in favor of a grapple attack unless you have a maneuver to push or knock her down!) and if you fail, not only will the hostage die, but a big baddie who you will need to fight will pop in.
And if you do save the hostage, perhaps the priestess can still choose to cut herself which will summon the demon that might well eat her before turning its attention on the party.
You can play around with that scenario a bit, but it leads to an interesting template for demon cultist scenarios.