So you are interpreting the lack of detail about how a Skeleton is controlled by its master in the MM to imply that it is definitely not as a bonus action?
We have since received statblocks that do limit monsters in this way. The recent summoning spells, the artificer creations, and a few others.
So yes, since the Undead statblock is no different from a goblins or a trolls, then I must assume they need no bonus action to be commanded. If they did, then an errata would be added to correct the mistake.
I'm really going to need a citation from the MM for this. I do not see where it states that you can give multiple commands in a single round as you claim it does.
I'm not saying you're lying. Just that I couldn't find it.
Zombie page says that they "can follow simple orders and distinguish friend from foe" and does not say that a horde of zombies all follow the same order. So, you can give discrete orders to them. Also, the identifying friend from foe part is also not in the spell.
Skeletons are actually more nuanced, because of their intellect. It says they can perform many tasks, mentioning things like scaling a siege ladder and forming a shield wall. The tasks need explained to them, which implies that MM skeletons can actually follow a set of commands that culminate in a single action, like loading and firing a trebuchet. But, since all of the actions listed seem to fall under sieging or defending a castle, and the entire entry only speaks of a single master, then it seems a skeleton can be given a command and left to do its work. Also, since they are being used in castle defense and offense, it makes perfect sense that they can have different groups doing different tasks, since everyone acting in the same manner is terrible for those sorts of tactics.
So, stated directly? No. Stated by context? Yes.
If that is your interpretation of it, then that is fine.
I'm going to need you to point this out for me as well I'm afraid.
You're claiming the spell gives guidance on what the skeleton does after you lose control and it stops obeying your commands.
I'm claiming that the spell does not.
If I missed something like that, that's extremely worrying to me.
"If you issue no commands the creature only defends itself against hostile creatures."
If we accept that you must command them not to attack the living, like the MM states, this is actually a problem. Because with zero commands, they do not follow the actions listed in the MM, they stand motionless and defend themselves.
So, we have two seperate end states. 1) The monster is hostile, needs to be commanded not to attack, and attacks immediately upon being free and 2) The monster is motionless, does not need to be commanded to not attack, and will simply defend itself even when freed.
I think that it is very likely that skeletons can be controlled verbally. Just not by PCs without DM intervention.
So, the monsters and NPCs act differently without DM intervention. Which supports the idea that the MM is giving us a different type of creature, because without the DM saying otherwise, they act differently.