These reasons do not in and of themselves make acid any more or less powerful than any other energy type (not that acid is an energy type per se, but neither is cold).Acid damage is more powerful for three reasons:
1/ Fire-based critters are far more common than Acid-immune critters.
2/ Acid damages objects (like Sonic damage).
3/ Acid is less common than Fire, and therefore counter-measures are less frequent.
That last one you can change for your campaign, but it will take some effort.
Acid spells are more powerful for yet another reason:
4/ Acid spells are typically of school Conjuration (Creation), and thus do not allow spell resistance. You can harm a Golem with acid arrow, you cannot harm a Golem with scorching ray.
Your reason#1: Frequency (or infrequency) of resistant creatures doesn't make energy more or less potent, it just means that; more or fewer creatures are resistant, nothing more. How you interpret that is up to you, but it doesn't make the acid any more or less potent.
Your reason#2: All energy types damage objects. And with dice used to determine damage, acid could just as likely do less damage than other energy types. While acid has the advantage of ignoring hardness, it doesn't ignite combustibles like fire does, it does chill objects or freeze liquids like cold does. Like anything else, it has its advantages and uses and its disadvantages and occasions where it is inappropriate.
Your reason#3: Acid may be less common than fire but that is because it is a chemical compound, not an energy type. Just because it is more complex a substance than fire, doesn't mean that countermeasures are less frequent in availability. If there are more countermeasures against fire, it is because fire is encountered more frequently and people choose not to have countermeasures against acid. Again, this doesn't make acid more powerful.
Your reason#4: This reason is easily corrected as acid was made conjuration just so Evokers wouldn't have all the good damage spells. Conjuration spells last longer and acid is more damaging to objects the longer it is around. An acid version of Fireball would be just as damaging as a typical Fireball. There is no reason that acid could not be used in more evocation spells. And vice versa, there is no reason a conjuration spell could not summon a fire arrow instead of an acid one or an electric one. Incindiary Cloud is similar in effect to Acid Fog and both are conjuration spells, but why not make an electrical version?
Acid in and of itself doesn't, effects created by conjuration magic do.I didn't realise that acid ignored spell resistance.
You're arguing against yourself. Changing something is not redundant. Making a duplicate of something is redundant. And a variant of a spell should be of equal power (albeit with different effects) not more or less power. A 1st level Acid Hands should do as much damage as a 1st level Burning Hands, just a different type of damage. The acid is no more or less potent than the fire, it just behaves differently and is useful under different circumstances. Acid hands is going to cause the same damage as Burning Hands and its not going to do anything else. Whereas Burning Hands has the potential to ignite combustibles that are exposed to it.I'm generally against making variants of an existing spell just by changing the energy type. It's redundant.