Only one.

The wording seems deceptive, but it says "For each spell", not each spell level. Your math is correct concerning DC of the checks, but dispel only requires one check, based on spell level.

1. Treat healing/round as damage/round for calculations

While the DMG doesn't give a clear answer, Spellcasting's effect on CR (DMG p.281) states:

\$\begin{array}{|l|l|l|} \hline \textbf{Name} & \textbf{Example Monster} & \textbf{Effect on Challenge Rating }\\ \hline \text{Spellcasting} & \text{Lich} & \text{See step 13 under "Creating a Monster Stat Block"}\\ \hline \end{array} \$

Where Step 13 states:

Innate Spellcasting and Spellcasting. The impact that the Innate Spellcasting and Spellcasting special traits have on a monster's challenge rating depends on the spells that the monster can cast Spells that deal more damage than the monster's normal attack routine and spells that increase the monster's AC or hit points need to be accounted for when determining the monster's final challenge rating.

This tells us that spells in general should influence CR (Unlike some other monster features), it's just lacking by how much.

We can however reverse engineer a monster in the book to see how its CR is affected by spells such as these, by determining its CR before taking into account healing and other beneficial magic, and comparing it to its actual CR. Let's take this Couatl into consideration. Most of its spells are healing and damage mitigation, and it gets a large amount of them, leaving it as a good starting point. Breaking it down based off the Monster Stats by CR table (DMG p.274), we have:

Defensive CR: With 97 HP* the Couatl lands it in the CR 2 category. Its AC of 19 is 6 higher than the expected CR, leaving it Defensively CR 5

Offensive CR: At 10 damage/round, Its offensive CR starts as a 1. Its attack bonus of +6 is 4 higher than the suggested, leaving it Offensively CR 3

Averaging these out, the Coautl comes to CR 4. This is without factoring in any amount of healing or beneficial magic cast.

The next step is to see how far we can deviate from this, to see if magical healing increases CR on top of normal calculations. The most apparent starting point is determining if we can raise it's effective HP based off of healing magic, and still stay within the confines of it being a CR 4 creature. Looking at the HP category it fell under, we see a range of 86-100 HP. Any more and we have the potential to bump up it's CR. Seeing that the Couatl is already at 97 HP, this means an increase of any more than 3 effective HP will cause the Defensive CR to increase to 6, increasing the Couatl's total CR to 5.

Since this is not the case, it can be inferred that healing magic does not affect the Defensive CR of a monster.

Now we see an example where a creature's CR is affected by its spellcasting. Take the Acolyte. With a defensive CR of 0 and being 27 HP below the threshold for increasing (Eliminating again the possibility of healing affecting Defensive CR, as using every spell slot to heal only achieves 24 points), the Acolyte needs an Offensive CR of 1/2 to meet its average CR of 1/4. This means 6-8 damage/round. Given it does 2 with a melee attack, 4 with it's only damaging spell, and heals 6 HP with a Cure Wounds, the Cure Wounds is the only option to put the offensive CR in line.

No examples given in the Monster Manual give creatures with the Aid spell, but due to it being a spell that's cast before combat begins, it stands to reason to work like Mage Armor (DMG p.276), in this event granting HP to go towards Defensive CR calculations (As there's next to no difference in a creature with 120 HP with Aid than there is with a creature having 125 HP without Aid). Whether it affects more than one creature (Potentially raising the effective HP boost to 10-15) is detailed below.

*Doubling it's effective HP, as the book suggests you should do for a CR 4 creature with immunity to nonmagical bludgeoning, piercing, and slashing damage among other immunities and resistances (DMG p.277), brings its Defensive CR to 10 after calculations, leaving the Couatl CR 7 by these notes. Why this step was omitted I don't know, but is beyond the scope of the question.

2. There is no clear answer

The only two Monster Features (DMG p.280) that revolve around area effects, the Breath Weapon and Death Burst, operate off the assumption that two creatures will be affected, so this is the best baseline to go by. It's unknown whether the caster should be included in the two creatures, or it should be the caster plus two creatures (Since neither given feature is sufficient to determine if the intent was to affect two creatures or affect two other creatures), so some discretion is needed.