So sorry that you are having such a rollercoaster with Ziggy, but glad he seems better at the moment. I would second what Karlin has said about medication - most neurologists don't now prescribe frusemide, not only because it can damage the kidneys but because used long-term it can eventually damage the heart. And the logical place to go if a large dose of gabapentin isn't really working is Lyrica.
To be honest, if Stephanie was my neurologist I would want a second opinion - which you have every right to ask for - and see if you couldn't take Ziggy to see Clare Rusbridge in Surrey. Clare is the leading researcher into CM/SM, and such a caring person, with a huge experience of treating Cavaliers. She doesn't come cheap, so I hope Ziggy is insured! But I don't think that simply increasing gabapentin and adding frusemide is going to get to the bottom of the sort of pain Ziggy is experiencing. Red eyes can be indications of headaches due to pressure behind the eyes from enlarged ventricles (the spaces at the front of the brain), and in my experience with Oliver gabapentin doesn't really touch it.
If you look in the In Memoriam section of the forum you will see my tributes to Oliver and Aled, who have both been put to sleep in the last 10 days