The restless dead include ghosts. If you can find the ghost of an animal, you can use this with it, too, if you like, provided the animal knows its name or you have an AC to the ghost.
I have no problem with using a fixed AC to make a kollossos, but you'd be making the connection weaker, not stronger, in terms of Penetration. To make a kollososs you need an item of the body which is an AC. A fixed AC of the body is, definitively, an AC. So, sure.
But the intent is: you get something "of the body" with an AC to your enemy, and by putting it in a kollosus, you fix that AC. This takes no vis, or study time, and requires no roll.
"Successes are automatic." The representation is about as life-accurate as a gingerbread man, so it doesn't require a roll.
No, it doesn't sum, but it doesn't cost vis or study time, so you can have billion of these if you want. They are weaker, sure...but they are essentially free, so there's a cost-benefit to consider.
The Leadworker fixes the AC by making it a kolloosus. That is, the process of fixing it and the process of making a lead voodoo doll are the same process, you don't do one then the other. It's not "no time", its "no study time". You can make them in about the time it takes to make a little human shape out of a lump of lead, which is really quick with tiny ReTe spells.
You can't revive an expired AC by craftin it into a kollosus.