(1) So you can shoot a wormhole to some distant planet, creating a sort of spatiotemporal bridge between two points allowing for effective teleportation without the destruction and creation of matter at its terminal points. At the point of the wormhole away from the origin — let's name it the "end" — you will not only be at a point in space howevermany light years away, but also [number of light years travelled] - [relative time to the wormhole as it travelled] Earth years in the future.
As a lay reader with some knowledge of time being relative, but being very hazy on practical ramifications, and more interested in being told a good story, I like this even if it is not the most scientifically accurate. The method by which this is achieved could also, in theory, involve some sort of exotic matter or state of matter, or energy, which may or may not actually exist, but this might veer too far into soft science fiction or science fantasy for what you desire. It would, however, possibly be helpful to the fiction itself, depending on what the desired result actually is. This would, I think, however, fit well with the "magic wardrobes in space".
(2) Wormholes near enough to each-other can create a cycle that crashes them, triggering some catastrophic meltdown or collapse into a black hole, making functional timetravel impossible, even though what you are doing in travelling through wormholes is functionally timetravel (one end might, for example, have a visually exploded Betelgeuse in the sky, while another does not), albeit timetravel that has no probable bearing on the two ends, being so separated by distance as they are.
Looping could, of course, be avoided by having two wormholes that lead to a certain place, but no local temporal overlap between them (one from Massotis to Earth which lasts from 1500-1850, but which is then destroyed, followed by a subsequent wormhole between Yttes and Earth from 1920 to the present day, with one having existed between Yttes and Massotis the whole time). I could see timetravel being functionally possible with some manipulation of this, now that I think on it, though it would require a great deal of planning and engineering, and ultimately likely not be extremely useful for anything specific.
(3) I had typed this before certain posts, which seem to corroborate what was originally a speculation that, looking through them, until the moment of the accident, light passing through a wormhole would function as an average layman might expect it to (that is, you would see where you expect to get to if you go through the wormhole); assuming that travel through the wormhole is slower-than-light in relativistic terms, I expect the "image" in the wormhole would remain thus till the moment of the accident, at which point it would cut to the elsewhere in interstellar space instantaneously. Depending on the scale of the image, you might also see the craft sent through arriving there (are travelling craft visible to a person outside looking into the wormhole?), and then presumably turning round as there isn't much point to remaining in the middle of nowhere.
This sort of disruption would (if it does actually happen in-setting) very likely be a minor risk every time one takes such transit, though presumably it would not be statistically more dangerous than automobiles (which are, to my understanding, actually relatively dangerous, especially when compared with trains, and sea and air transport, though catastrophic failure of any of these other methods is immediately far more tragically catastrophic because of the size of the crafts and the number of people likely to be involved). The shift in terminus, if it is unpredictable, could end up somewhere fatal, but my understanding is that most of the universe is empty, so it is more likely to simply end somewhere inconvenient but without much other consequence other than the loss of time (which may be very consequential in and of itself as far as individual humans are concerned, of course). Such transport vessels would, consequently, I imagine, have to be loaded with enough supplies for a round trip, just in case. It could even become proverbial for advanced preparedness for a wildly improbable scenario (if it is wildly improbable, that is).
(4) If you send something through a wormhole, the sending end increases in mass by the amount sent, and the exiting end loses an equal amount of mass? This is rather what it sounds like, however this is rather perplexing. My mind wants to conceive the hole as a rather literal hole, as a sort of "tunnel"; when passing through a tunnel in the ground, one does not increase the mass of the tunnel, nor reduce it, but rather an object with its own mass and such appears to pass through it. This is where the suspension of disbelief in a layperson might begin to break down: if this were a magic system, I would view it as needlessly confusing and complicated, though I suppose it does provide an easy way of closing a wormhole at one end if desired (simply send through a massive enough object and your hole is closed), however this also raises the question of what happens when one sends through an object that is too massive for the wormhole to handle. Does it simply collapse then? In this case, wormholes are extremely dangerous to maintain, and whether or not Yttes would have much popular support for their maintenance (dozens of potential black holes surrounding the capital of your interstellar empire is not really an appealing thought if you are a citizen of said capital of said interstellar empire, and the empire, in spite of the name, does not appear to be autocratic enough that they would maintain such things against the popular will); my guess is that this isn't what actually happens (perhaps the overmassive object is simply ejected back whence it came?), however it is something worth addressing, I think.
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This is not, of course, intended to dispute any actual science that would contradict what you already have, more to give an approximation of how it would be received by a lay audience more concerned about reading an interesting piece of fiction.
