Wouldn't the implied general rule of "you must have a caster level this big to cast this spell" be trumped by the specifics of Versatile Spellcaster? Otherwise things like Precocious Apprentice wouldn't work at all.
No, the Feat's rules are totally ambiguous to start with, but do make a tiny assumption which I'll cover in a second. The first obvious question you would have after reading it is do you need the ability to cast 4th level spells to do this?
The FAQ comes in and to a Metamagic based question (think an extended spell) says it's possible to cast higher than normal. However, this is a Metamagic based question. To understand what I mean, take for example an 8th level Sorcerer & Empowered Fireball. That's a 3rd level spell using a
5th level slot two 4th level slots, that deals 8d6+50% damage. Do you see the number dynamics there and where Versatile Spellcaster plays in? You can obtain CL boosts readily enough, but it's just as important as knowing a spell which tends to be the primary element discussed.
You're observation on PA is a good call out, but mindful actually doesn't support VA and opposes the FAQ ruling. See, PA calls out:
Until your level is high enough to allow you to cast 2nd-level spells, you must succeed on a DC 8 caster level check to successfully cast this spell;. So in it, PA creates an exception to cast a 2nd level spell as say a 1st level character by using the provided rules.
VS on the other hand only creates an exception toward using the spell slots. And it does so in the operation that kind of assumes that you already have the higher spell level: unknown-level-sorcerer combines two 2nd level slots for a 3rd level spell they know, specific builds/combos are almost never used to this would mean the assumption is the sorcerer is of at least 6th level.
When comparing the two, you'd actually see that VS shouldn't break any sort of level cap to begin with. However, the FAQ that everyone hates for nerfing their favorite trick did broke it for power, sort of ironic I guess.
Also, helpful link
here (got a newer link?). Mindful, you still need some sort of access. So we can move on to talking about how you'd need a Runestaff to merge two 2nd level spells to cast Animate Dead (a typically 4th level sorcerer spell).