You're welcome. I like problem solving and this way I don't have to do any of the graft.
One of the other issues with a battery being left for so long is that the electrolyte tends to stratify which won't help matters. I suspect that to rejuvenate the battery is going to take some time and patience.
The way I see it, you have two distinct problems, possibly three.
1. Battery is DAAD
2. Volt Save thing may be dodgy but modus operandi definitely unknown.
3. Volt Save thing might be fine and you have some other problem.
If it was me I think I would bite the bullet and remove the seat and:
1. Get battery in house for charging and voltage reading at regular intervals. Give it some gentle shaking, movement periodically.
2. Borrow substitute known good battery, drop in and see if things work. Just use jump leads to temporarily connect if the terminals are incompatible (but don't leave unattended - sorry for the egg sucking reminder)
3. If they don't work, Pred's check for the earth would be a sensible and quick thing to do.
4. If that makes no difference, with the seat out you should have a better chance of figuring out the wiring.
Let us know how you get on.