As you said the dark secret revelation is going to add some illusion spells, but it is very limited in which spells you can add. This is going to be more useful at higher level when thing like shadow conjuration and shadow evocation come online. I think focusing solely on illusion is going to be a mistake especially at early levels.
What I would do at low levels is to focus on summoning spells. Take Army of darkness as your first revelation. This will allow you to take Augment Summoning without having to take spell focus conjuration. It also allows you to take other summoning focused feats Including superior summoning. It also allows you to add the shadow creature template in addition to the celestial and fiendish. Being able to summon up flying creatures that can drain STR is actually pretty good. The augmented summoning will also work to boost your shadow conjurations if you summon a creature.
There is no reason you cannot go for a dual focus. Start with the summoning and when you get higher level you start focusing on illusion. That is going to give you a lot more flexibility which is the real secret of being an effective character. Summoning does not require much investment other than augmented summoning and maybe superior summoning.
Another option if you want to focus on illusion is to change your curse to haunted. This will give you Minor images at 5th level. That way you would at least have one general purpose illusion spell.
As you said the dark secret revelation is going to add some illusion spells, but it is very limited in which spells you can add. This is going to be more useful at higher level when thing like shadow conjuration and shadow evocation come online. I think focusing solely on illusion is going to be a mistake especially at early levels.
What I would do at low levels is to focus on summoning spells. Take Army of darkness as your first revelation. This will allow you to take Augment Summoning without having to take spell focus conjuration. It also allows you to take other summoning focused feats Including superior summoning. It also allows you to add the shadow creature template in addition to the celestial and fiendish. Being able to summon up flying creatures that can drain STR is actually pretty good. The augmented summoning will also work to boost your shadow conjurations if you summon a creature.
Don't think so.
Army of Darkness enable switching celestial/fiendish template with shadow template. Not adding
And shadow template doesn't give STR dain attack.
If you are an Elf, you can take the Ancient Lorekeeper Archetype, which allows you to pick your mystery spells.
11+ year necro, pretty good. Although I'm not sure the OP is still looking for help on this.