It is worth bearing in mind that the Monster Manual Variant Spellcasting for dragons is a DM option that lacks a great deal of specificity. The Monster Manual is a DM tool, strictly speaking, and the DM has considerable latitude in how to implement a dragon's spellcasting ability if it is chosen. Interestingly, the A non-variant dragon does not have the Innate Spellcasting trait (point made bythank you @Spitemaster)
A young or older dragon can innately cast a number of Spells equal to its Charisma modifier. Each spell can be cast once per day, requiring no material Components, and the spell’s level can be no higher thanthe spell’s level can be no higher than one-third the dragon’s Challenge rating (rounded down).
At first blush, casting the spell at its lowest level isn't a restriction. Since both of those spells can be up-cast per their spell text, and the dragon has the capacity tocan cast spells at third level based on CR/3, the dragon could cast them at up to third level. (Up cast them). Does the DM want to treat the dragon like a warlockWarlock? There isn't guidance on that one way or the other, so the DM can treat all of this dragon's spells as third level spells.
This text suggests that the invisibility spell would need to be cast at 2d level and could not be up cast.
Is there a loophole? Maybe. You can argue that there's a 'noted otherwise' in the dragon's variant ability, and that @spitemaster's point on thethus innate spellcasting is not being a hard restriction applies.
the spell’s level can be no highercan be no higher than one-third the dragon’s Challenge rating (rounded down)
ThisThe restriction is "no higher than." This can be looked at as specific over general (Basic Rules, p. 5) whichthat opens an exception to the general rule since it only limits the dragon spell levels at the upper end. You can also make the case (as Szega does) that this isn't specific enough to override the general innate spellcasting rule. (A fair ruling).
That leaves open to aDM ruling whether spell slots are even a thing for a dragon. If they are not, then dragon'sdragons are only limited by their CR for whatthe level a spell is cast at which their spells are cast. For example, are the spells cast as a warlock's spells are cast:, all at the same level? That's up to the DM to decide.
When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. For instance, if Umara casts Magic Missile using one of her 2nd-level slots, that Magic Missile is 2nd level. Effectively, the spell expands to fill the slot it is put into. (Spell casting, chapterChapter 10)