Net, some of the technology you're developing won't work very well, but you can also build it and have no IC reason not to try, so I'll let you find out by trial and error when you actually need to use them. The same applies to everyone else.
Spark can't be everywhere at once. The guards are pretty savvy themselves, and there's a wall around the fort's only entrance from the surface, so it's not like they've got a great likelyhood of getting through anyway, but still. Aside from that, the little thing is far from indestructible, and far more useful as a research assistant than a defense station. Incidentally, you don't have unlimited communication range with it, that would be rather more powerful than I'd like, there's a reason even Mary Sue-writers often place a range limit.
When sending spells over a long distance through normal space, or a really long distance through abnormal, there is, in fact, latency, because the transmitted energy has to travel like anything else. However, provided the spell is transmitted using anything even approaching the speed and efficiency of a hand radio's signal, that lag is barely even relevant within planetary distance, measured in minute fractions of a second.
There is an increasing cost based on distance when casting spells, because you need to keep the construct together over the travel time. By the time major lag became much of a problem with radiofrequency-equivalents, you'd already have reached the limit of your personal ability to project the spell over range without a ritual to back it up, which takes some time to set up, or in cases of lag greater than a few seconds, of simple possibility. Of course, casting to something more like the speed of sound, or the flight speed of an unladen african bush swallow, will render lag far more of an issue, while simultaneously opening up a number of means to cast over far longer ranges without needing to use the more difficult method.
There is a more difficult method to use magic over extreme distance. Basically, you create a gateway to a semi-nonexistent chaotic plane, then another gateway to the target set of coordinates, then send the spell through. Doing so is extremely difficult and energy-intense, making it only worthwhile in exceptional situations, but also massively increases travel speed, to the point of hyperluminal transit when used correctly. Can be dangerous to get it wrong, and there are theoretical applications that should be looked into, by the suicidal, desperate, or very brilliant.
Net, you got a great deal wrong with your textbook post, but suffice it to say that yes, you do have to consider the shared frame of reference. Provided you are capable of reconciling extra-dimensional coordinates relative to a subjectively stationary physical object within a short timeframe, doing so is not particularly difficult, but the different contextual and semi-arbitrary layers tend to cause problems for most things running on a human comprehension-scale. Having servant-devils or similar entities is often helpful.
I am not actually certain I answered everything I need or want to answer, so feel free to re-ask any points you feel I missed.