"Only damage taken while wearing the ring" does not imply that the regeneration only heals damage done since the last time the ring has been worn.
Let's imagine I'm a character wearing a ring of regeneration. I get dealt some damage, let's say 20 damage, and my ring heals me, round by round, 10 hit points.
Then I remove the ring and I still have 10 damage. I get dealt 3 more damage, bringing the total to 13.
When I wear the ring again it starts healing 10 more damage, bringing me back to 3 hp lost.
What happened? Only damage taken while wearing the ring is regenerated holds true.
So, you lost your hand and we decided this amputation is considered to have been done while wearing the ring. Next time you wear the ring, no matter how much time has passed, no matter if you lost both hands and now are in need of an Hand of Glory to wear it, you start growing back your hand, period.
If the hand gets pressed against the stump in time, instead, I would say the hand would get reattached and the ring would keep working once the hand is attached again.
While not attached, I'd treat the hand as an item, who does not regenerate.
If you can meet the prerequisites of prestige classes by your previous gestalts, than it is reasonable to take Beastmaster as soon as possible. I assume you are going to finish your Convoker advancement, so the character would look like that:
Commoner 6 // Convoker 6 // Beastmaster 1
At the first level Beastmaster provides boost to your effective Druid level, so it will be a companion of 4 level Druid from the spot. I dont realy think you'll very much need any more levels in Beastmaster as it stacks with any companion advancing classes, and they have better class features.
From now on I see two options:
- Take Druid for spellcastng, if you have room for it in your actions. Five levels will give you access to highest level spells even without that level lost due to Beastmaster. You also will have room for adding more prestige classes at those five levels later, as Druid is a regular class. Finaly, as a regular class, it has no prerequisites. It results in:
Commoner 6 // Convoker 6 // Beastmaster 1/Druid 5
- Stack some other prestige and maybe regular classes with nice class features wich progresses your animal companion. I'd consider Wild Plains Outrider, good class with quick advancement and ability to make a full attack after mount's single move. Two remaining levels may be of practically any class wich progresses your animal companion, probably those of a Beastmaster or even some regular class whose class features you like. It results in:
Commoner 6 // Convoker 6 // Beastmaster 1/Wild Plains Outrider 3/xxx 2
I can't realy wrap my mind around Animal Ally feat mentioned by Chemus. I feel like, depending on your DM's judment, it may be quite usefull, but when to take it is a question I can't answer. Beastmaster and Wild Plains Outrider are both from Complete Adventurer.
There are no official rules in D&D 3.5 for dismemberment in combat.
There are specific cases where it can happen (like the Vorpal weapon property mentioned in the question, the Wu Jen spell Decapitating Scarf (Complete Arcane p. 102), or the removal of a character's hand to make room for the Hand of Vecna), some of which provide rules for their specific case.
There's also a variant rule described on p. 27 of the DMG called "Damage to Specific Areas," but it doesn't really cover how dismemberment would work, it just suggests penalties to apply if a character, say, steps on a caltrop, or has their fingers slashed by a trap.
Since you mention being okay with third-party solutions, you might consider: