Could ALSO be true (don't think the two are mutually exclusive) as Edea/Ulti speach after killing Deling seems to indicate she's been persecuted.
Certainly, Rinoa might have been driven mad because of this persecution. In fact, that's basically what the theory says(or what it should say, I might add).

But is the 'sorceresses age'thing the only thing that makes it impossible? (as opposed to 'possible but no reason to believe it?')
Yes. All other things like Rinoa and Ultimecia in time compression together, Ultimecia looking different than Rinoa, and stuff like that all have their reasonable counters. However, when we get down to it, Ultimecia lives hundreds of years into the future, so for the theory to be possible, Rinoa must have an extended or infinite lifespan(while she is a sorceress). There has been much dispute about this, but hopefully the Ultimania will make the answer obvious.

1. She would have known about the trap.
Not necessarily. Although it may have been written down in history that Laguna trapped her, I personally doubt that Ultimecia would know, or remember, such a small fact.

2. After the trap, if she would have just checked in on Edea about 5 years after the incident, she would have had Ellone almost literaly right under her nose. (As in - trying to controll the other sorceress may have been a good idea)
But is not that what she basically did? Edea realised what was happening, and had Ellone sent off with the White SeeDs, telling them to stay away from her, and then she let herself be possessed.