Does the gun fire the bullet or the person pulling the trigger?
A gun doesn't shoot by itself. A ring of spell storing doesn't fire off a spell on its own. To call a non-sentient ring of spell storing a "caster" is an illiteracy.
Perhaps a better analogy is a tape recorder.
I'm going to Russia. I can't speak Russian, so I ask a native-speaker of Russian to record a number of helpful phrases in Russian: a proposal for romantic activity, a request for directions to the nearest toilette, some profanity to cuss someone out, a request for medical attention, etc. I have buttons on my tape player that I can press to play the phrase I want.
If I play the recorded romantic proposal to an attractive woman, who turns out to be married, is the husband going to blame the tape recorder, the original speaker, or me—the person who pressed play while directing his wife's attention to it?
In the tape recorder example there are two communicators. The first is the Russian speaker who voiced the words, which were captured by the recorder, but not yet delivered to a recipient as a meaningful communication. The second is me, who pressed the button to play those words in a certain context, directed at one or more specific recipients. I'm not able to voice the Russian words with sufficient fluency to be understood, but am able to press a button and deliver a message.
Similarly, the original spell caster is fluent in a specific spell that I do not know. She casts it at the ring, which stores it. But the spell has not been directed at a target and has not impacted the world at this point, beyond being stored. It requires a second caster, who need not be fluent in the spell, but only attuned (i.e., knows how to use) the ring. The second caster selects the time, place, and target (if applicable) for the spell.
The temple curse discussed above should clearly affect the second castor, as THAT is the caster who cast the spell within the temple. There is some room for argument that BOTH casters may be affected (I wouldn't rule that way), but it would be illogical to claim that the second caster isn't a caster.
If the second caster has class abilities that can affect the spell, I would rule that his abilities apply to the stored spell when cast from the ring, even though the spell level, saving throw DC, and attack bonus is that of the first caster.
To beat a dead horse further:
- The ammunition manufacturer creates the cartridge, which will determine the velocity, penetration, expansion, and other aspects of the bullet's performance. The shooter provides the aim and the decision to pull the trigger.
- The speaker provides the language fluency, as well as the pleasantness, variation, and clarity of voice. The player provides audibility (via volume controls, selection of environment, etc.), audience, and venue.
- Caster 1 gives the spell power. Caster 2 decides the time, place, and target at which the spell is released / fired / cast.
TWO casters. It is RAW and, in this case, I strongly agree with the RAW.
Of course, your DM may decide otherwise. This may be either because he suffers from poor logic and poor English comprehension, or because he simply wants things to work different in his campaign. Either way, you just have to roll with it.
It is rarely going to come up in a game and when it does . . . well, few DMs are going to give it the amount of thought we have in this thread.