From a pure balance perspective, I don't think you should do the spells for critical trade.
For a ranger, casting is a buff & control prospect. They don't get that many spells per day, and they don't generally form the basis of their offense (Mark of the Hunter excepted).
A character will get more use out of an automatic critical bump than they will a situational spell toolbox. It would also make the class less versatile/fun.
If you look at the Oathbreaker Paladin, the new abilities are a 1:1 trade; new spells, new channels.
I think your approach of moving towards necromancy/illusion is a good one.
I would consider adding some Warlock flavor in place of the ranger spells and some of the spell-like abilities; that will definitely jibe with the Asmodeus lineage.
One other aspect of the Oathbreaker, it is really just a different 3rd level character option; it just happens to be an anti-paladin.
I'm not familiar with BGEE or the Fallen Ranger; nothing about the Hunter archetype, in particular, is necessarily Lawful or Good.
A Fallen Ranger to my thinking would be one that turns on the natural world, or becomes urbanized and remote from it.
That is to say, Rangers aren't necessarily wholesome to begin with in the RAW.
You could lean into the fiendish aspect and do something like add Pact of Chain flavor; it's the most ranger like.
The Revised Ranger features a better Beastmaster option; it adds a companion who scales with the Ranger level and the Ranger gains no other Archetype features for herself.
You could assign a lesser demon as a retainer to the Ranger; they could be a corrupting influence sent from the Abyss, charged with ensuring the Ranger is fully corrupted. Demons work well for this mechanic, as they change forms as they become more powerful; I don't actually like the idea of a 17HD panther, but that's me.
Aside from that, favored enemies could be changed to human/humanoid; that's pretty Fallen.
Primeval awareness, that could make them a manhunter; having their abilities changed to something clearly suited for ill & evil would be a bit of a punishment.
Just some thoughts. Again, Rangers aren't wholesome to begin with.
For me, I love to have a band of Lizardfolk rangers tracking the party through landscapes. They're certainly evil, yet natural.