Hi Sam,
Did you have a sense the seizures were related to low cardiac output? Anything in the history to suggest that? It's sad the dog has so many problems at such a young age.
Unless we're sure that's not the case, then improving peripheral circulation stands to benefit all three complaints - OA, heart failure and seizures.
A Yin tonic like you propose can do that in general. If the pulse was also rapid, and there is tachycardia accompanying the heart issues, then we can try Zhi Bai Di Huang Wan to slow the heart rate due to its berberine content, while resisting hypertension.
If the heart rate wasn't especially fast, and if the tongue had a darker red tint, then I'd look at Xue Fu Zhu Yu Tang to mobilize peripheral circulation, and support the Blood and Kidney Yin.
If the seizures aren't heart related and show any increase in the weeks ahead, then we'll have to re-evaluate the dog and try to roll seizure control in there. Otherwise I'd start with the above, given what you've described.
All the best,
Steve