I understand completely what you're talking about as far as that specific frustration of not knowing any detailed or even a very accurate history of a Rescue-Bird's prior ownership, age, and medical/health history...We try very hard at the Rescue I work at to try to get as much information (and more importantly documentation) of all the bird's histories, but most people are not the original owners of the birds they are surrendering, a lot of people just flat-out lie, and sometimes it's just impossible to know anything at all about the birds. I try to not write-down anything that I can't verify myself through wellness-exams, blood-work, behavioral tests, etc. about the birds we have up for adoption, but it's hard...
As far as parrots and seizures go, a lot of the time they unfortunately end-up being diagnosed as "Ideopathic", meaning we don't have a clue why it happened, and it might happen again or not happen every again...That's frustrating, more frustrating than a lot of much more serious health conditions even because at least we can diagnose the cause of them and know exactly how to treat them and prevent them...With seizures, it usually doesn't work that way, especially with birds. There are a million different things that can cause a bird to have a seizure, and a lot of the time it never happens again, and the poor owners are just waiting and waiting and waiting for it to happen again, and sometimes it never does, which is a good thing, but still horribly frustrating...
I know you have an AWESOME CAV, so that's a big plus...Usually when a bird has a seizure and it's thought to be related to a heart issue or heart "disease" of some kind, it typically has something to do with the heart not being able to pump with enough force, so fluid in-general builds-up in the body, and a lot of the time the seizures actually happen as a result of a sudden change in the bird's position...For example, if the bird has a heart condition that makes it difficult for it to pump enough volume or with enough strength (a lot of murmurs are a tip-off of valve issues that cause this weakness), and fluid starts to build-up as a result of this, when the bird has been in one position for a while, say while it's sleeping, and then after being in that position for a long period of time they suddenly move, like they stand-up suddenly, or they suddenly leave a perch they've been sleeping on or standing on for a long period of time and they fly somewhere, that sudden shift in position causes a sudden, drastic change in blood-pressure that the bird's heart cannot compensate for, causing a seizure (Orthostatic Hypotension)...And once the bird's blood-pressure finally adjusts, the seizure stops, and often won't happen again until the bird is again in one position for a long period of time and then suddenly moves, causing their heart to not be able to compensate and their blood-pressure to suddenly drop again, causing another seizure. There are a lot of different forms of this same issue, and a lot of different heart conditions/issues that end-up causing this same issue with compensating for sudden blood-pressure changes, which result in a seizure...Usually the treatment is medicinal, and usually very simple and effective, such as prescribing a daily Diuretic such as Lasix to keep the fluid from building-up in their bodies due to the lack of strength of the heart...Sometimes they also add a Beta-Blocker, Alpha-Blocker, Calcium Channel Blocker, etc. depending on what the actual issue with the heart is, but typically when seizures occur it's almost always a fluid-retention/blood-pressure issue due to a weak or leaky heart Valve which causes Orthostatic Hypotension, and all that is needed is a simple Diuretic...I hope it's that simple for your Frankie...