The rule on positions and placeholding is not that complex:
Whoever you appoint to a position is the rightful holder of that title/position. They are not a placeholder, temporary, or benchwarmer. The position belongs to them. Whoever held it before is no longer the rightful/proper/real/etc. general/duke/margrave/etc. They lost the title. It is not theirs.
The bottom line is: If you appoint someone with the intention that they will only hold it until "the rightful office holder returns", then you are appointing a placeholder, and that is against the rules. Doubly true if you actually *say* that you are appointing this guy temporarily until the rightful guy returns.
That's pretty much the end of it. The rest of this post is all about how to handle the situation when it rears its ugly head IG.
This problem usually only crops up with positions that are normally appointed. There is the expectation that you are appointing a new, permanent holder of the office. In positions where the office is elected on a regular basis, and you choose to appoint before the election runs or completes, then the expectation is that they will hold the position for the remainder of the term. They may choose to not run in the next regular election, or they may choose to run and then lose to the previous office holder. That's fine, as that is the nature of an electoral system. It's a great benefit of that kind of system.
There are a couple ways to deal with this situation in the context of the game, without breaking any rules/policies:
1) You can appoint someone who wants to keep the position for themselves. Problem solved.
2) If the original noble lost the position due to getting captured/wounded, then just wait for them to come back. You probably won't get any warnings about empty positions for a couple days. You shouldn't run into any problems even if you let it stay empty for a week.
3) If they lost the position due to inactivity, then you can allow the position to stay empty long enough to see if the original noble autopauses. If they do come back before they autopause, and the position is still empty, then you can freely re-appoint them. Again, a delay of a week or so shouldn't cause any problems. And if they autopause then you can probably assume they're not coming back.
4) If you have a realm that is in the habit of voting on a lot of things, then toss it up to a short referendum or ad hoc show of hands. "Wait for Sir Kepler to come back, or find a new Duke?"
5) If the position is a regularly elected one (monthly or quarterly), then let the system take care of itself and do what its supposed to do. At the next regular election cycle, the current and previous position holders can run or not as they desire.
As far as council positions and even lordships, the only time you are ever going to have a problem is if you have a system where the position is "elect once when empty", and you have a habit of usually wanting to put the old guy back in place without really doing an election. In that case, you should probably rethink your system. What's the point of having an "elect when empty" system, when you only want to actually have that election under limited circumstances? What may fit better is have it normally appointed, and then hold a manual referendum in those few cases where you want it to be elected.
