Sorry, I forgot about the use of the amulet in multiplayer games. More broadly, the overall function is to give the PC a means of summoning the Shield Guardian as many times as needed, as long as the SG never dies. In other words, if you lose the SG because he is stuck (or left) on a different part of the map, or he isn't there when you start the next module, or some cut-scene disappears him, or you log off in a multiplayer game, or some other issue, you can can use the ammy to get him back or call him home to the PC. But, by Bioware's design, once he actually dies, the ammy is gone, along with the ability to summon him again.
Actually wearing the amulet is only necessary to summon the SG in one of those situations or to gain its AC and reflex save benefits. In fact, because of the way the amulet is scripted (where little attempt is made to distinguish between the activation, acquisition, equip, etc. events), it's not always even necessary to activate the amulet to summon the SG. Often, just taking it out of a container or equipping it will call the SG. That's actually what usually calls him the first time, when he is first created in the Maker's sanctum and the PC acquires the new Controlling Amulet (which is a different amulet after the machine is used than is found on the earlier level of the ruins).
There is no change in the SG's behavior when the amulet is worn. Despite any implication in the section of the description you quoted, it doesn't change his AI at all after he is summoned. It's worth noting, though, that if you remove him from the party (say, for XP purposes), his normal faction is merchant, so it's possible to turn him hostile with friendly fire. The version I cooked up is of the defender faction by default, to avoid potential hazards that can largely be filed under the heading Robe of Balpheron. ;-)
The second part of the description you quote is, I believe, intended to refer to the ability to summon the SG after the PC has arrived in Cania. Any other henchmen can be brought in via dialog with the Reaper, but the SG is retrieved by the amulet.
As far as limited uses goes for any revised SG/amulet system, I was actually thinking of scripting limited charges and not using the normal BW item charge mechanics for that. For one thing, the electrifier is too obvious an exploit. But, even beyond that, we don't want the amulet to disappear after the SG is summoned for the last time because we might still need to call the SG to the PC if he is separated for some reason other than the SG's death. So, using that final charge has to be done differently.
Expanding the spell-storage options isn't a bad idea, but maybe not one I would want to do. The scripting BW put together for that purpose isn't especially flexible, IMO, but, they also haven't given us great tools for examining arbitrary spells that a caster has available without cycling through every sorc/wiz spell at that level. Honestly, it's certainly doable, but I wouldn't really want to muck with it. Additionally, there would be a lot of judgement call decisions in expanding the spells storable on the SG to include higher level spells, calls about high-level spells that have much more impact than level 1-4 spells have. For one thing, lots of higher level spells (like Premonition, Mass Haste, etc.) would be duplicating defensive properties available to the PC at golem creation. Also, lots of those higher level spells aren't intended to be usable on henchmen (they are caster-only spells). For example, should a mage be able to let his golem cast Shadow Shield? Giving an associate a +5 natural AC bonus is kind of a big deal and something mages can't otherwise do. And, at least for mages who are level 17 or higher in caster level, allowing the SG to store those higher-level spells isn't really making the SG scale better, since those spells are available when the PC first gets the SG anyway and stored spells won't be cast at a higher level from the SG as the PC advances. (BTW, the latter bit about the level at which stored spells are cast is true even if the mage was only caster level 5 when he made the SG,)
BTW, another potential option for scaling the SG would be to make sure the SG is level-able as I have done, but also allow the PC to choose (most easily and perhaps logically at creation) a couple of leveling options. Perhaps the SG levels with some other leveling package that emphasizes offense or defense or some special gimmick. When I created the SGs construct leveling package, I was interested in tweaking the standard creature leveling package to give the SG a minor offensive boost, but also toughening up an epic SG a bit with some extra HP and extra AC.
I could probably just make the files I changed available in case anyone wants them. There is only a custom HAK (I used the cep2_custom.hak file since I have CEP installed, but there is nothing in that HAK file specific to CEP) and an ERF to import into the 2nd and 3rd chapters of HotU. I think most players would think it's an improvement and more playable than the default version. Of course, I may have unconsciously tailored things to my goal for the SG, which was that he tanks better as the PC advances, but that he's still similar to the SG that BW designed, basically a big iron lug and not a dexer or tumble machine.
