looking at M152 there seems like a natural concern that someone who wants to hijack your zombies could just "brute force attempt" with Control Zombie until they succeeded.
It only takes 1 second to cast, and it's possible someone might just try this every half hour (regenerating 3 FP in between) until they succeed in the quick contest.
To have better chances in the "Quick Contest of Spells" what sort of thing (other than staying alive and within 100 yards) might a caster do to improve their chances?
M14 says "
A spell resists using the caster’s effective skill when he cast the spell" so it sounds like you'd want to pull out all the stops (Ceremonial Magic, pump in extra energy for skill bonuses) when initially casting zombie to get the highest possible effective skill you could.
It also means (a general note for Counterspell and Suspend Magic too) that any time you cast a spell you should keep a record of the "effective skill when cast" number until that spell is over, since that could come up.
If someone succeeded in using Control Zombie (they become the new master) and a 3rd party wanted to get that zombie and ALSO cast Control Zombie, it makes me wonder how you would resist in that circumstance:
1) do you still roll against the effective skill of the original creator, so new master's competence doesn't matter?
2) would you roll against the effective skill of Control Zombie instead?
If it's #2 then it would actually seem like a good way for zombie creators to guard their zombies would actually be to "steal control from themselves" by casting Control Zombie to "gain control" of a zombie they already control.
The reason for that is because it's much easier to pump up the power via Ceremonial Magic, since Control Zombie has a lower energy cost (3) compared to creating the zombie in the first place (8).
To get +4 to skill for +100% energy for example, would only cost 6 energy for Ceremonial Control Zombie, so you've spent a total of 14 towards that zombie. Getting that +4 to skill by using Ceremonial (Create) Zombie OTOH would cost you 8 extra energy (total of 16) which would be more expensive.
Not to mention that Ceremonial Magic's time demands are much worse, you're talking 10x 1 minute (10 minutes) whereas creating a zombie in 1 minute and then "Ceremonial Control" just tacks on a paltry 10 seconds, so it's also faster to make "skill-boosted" zombies more resistant to others' Control Zombie interference this way (assuming of course that you can actually resist Control Zombie with a casting of Control Zombie... I'm not sure if that's legal)
This is even more important if you rely on Mass Zombie (M153) since that has even higher energy and casting requirements.
Another benefit is you can make a magic item to give yourself +2 to Control Zombie, but there's no item like that for creating them.
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Spellguard (M127) doesn't mention being able to use the bonus it provides against Control Zombie (only meta-college spells), but would that seem reasonable? Should we have a "Zombieguard" variant of this?
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There doesn't seem to be any way of finding out if someone has stolen control of your zombie (until it ignores/disobeys, at least) because it's not like you'd detect loss of control as a spell: it's a permanent spell which doesn't count as "on" (you don't maintain it)
Is there maybe some spell anyone can think of you could cast on a zombie which would alert the caster:
1) if someone attempts Control Zombie on it (succeed or fail)
2) if Control Zombie is successful on it
Without something like that, I'd be worried about some mage slowly sneaking around wresting control of your zombies without you being aware of it, like they might not give any new commands (things go on as usual) until they've turned enough to overpower you.
I'm thinking you could "Analyze Magic" on your zombies to see if they're under the effect of someone else's "control zombie" but that would require constant active scans, so I'm thinking something a bit more preparatory.