The forums say to take Arcane Disciple to get 1st level spontaneous caster spells. The text of Arcane Disciple states you add such and such spells to your list and you can cast them using your Wisdom for the spellcasting purposes.
However, I disagree with what the forums are stating allowing Nar Demonbinder to allow one to qualify for Ultimate Magus:
At first glance it appears you can’t enter UM with Nar Demonbinder since it first level only gives you 4th and 5th level spell slots with spells you must select from a specialized list, and this list has no 1st level spells. Thus by itself you can’t cast 1st level spells and no UM for you :( . But this all changes with a feat that grants spells, such as Arcane Disciple or Mother Cyst.
The spell slots are simply not there to allow 1st level casting.
I explained to the player, if an 18th level Bard took Arcane Disciple, he doesn't have the spellcasting slots to cast his 7th+ level spells that the feat would give you.
Am I wrong? He is insisting the forums would not have stated otherwise. I have never known, nor have I personally, tried this class combination/prestige class before. I told him I would consult RPG.SE before giving him a definitive answer.
I stressed the point that I believe I am correct. But I have been wrong before, and I like to give my players the benefit of the doubt.