Land set to 'no build' + 'no object entry'...but still OTHER people's objects on it!

If auto-return is actually set, this shouldn't be possible, with or without object entry. Even if the sim has had a glitch with this (which I've seen maybe once), if you can find the objects and click on them, they should disappear within the auto-return interval. If that's not happening, I think you've found something worth a support ticket. Anyway, it's good that you've gotten Auto-Return set up--I think it will solve all your problems once things are working right--maybe the sim just needs a restart or something in this immediate situation. (Disabling object entry is almost never useful for anything except testing physical scripts or other temporary tricks to control building within a parcel surrounded by no-object-entry land. It's very handy for those advanced uses, but usually counterproductive for controlling prim debris. And disabling scripts, well... if you're really a griefer target, yeah, this can be useful, but for most purposes this is also more of an annoyance than a boon.)

You should really check your actually doing what you think you are, once being an estate manager I had to instruct a lot of people how to change land permissions, quite often I would run through with them what they should do and they would say done, but no they hadn't done what i asked, they had clicked something else completely. The worst people where the ones who said I've done this before I know what I'm doing.

We had a no auto-return policy and I always mentioned it as least twice before handing land over, "DON'T TURN AUTO-RETURN ON" "LEAVE IT AT 0", ten minutes later, I would start getting "you objects have been returned messages", However I must be honest, this is exactly what I did on my first day too

Auto return never returns prims that are either selected or sat upon so there's really no such thing as a "non-vehicle friendly" auto return time.

Vehicles don't add to a parcel's prim count do they?
I mean vehicles that enter the parcel (ie. being used)

There's a bug that makes auto-return and no-object-entry fail... Banlines and auto-return are the only things that keep 100% of objects out... At least I think they do

no object entry is not an absolute like a ban list, go up high enough and it doesn't have any effect.

Yes, they do, and I've been ejected from an airship and had it returned due to "parcel is full"

I wish I could find a definitive complete list of which parcel restrictions apply at what altitudes. I know I've gotten no-object-entry errors while building way above 50m AGL (the limit of whitelist avatar bans--I think). It was convenient for me that it worked that way, actually. I just don't think I know which limits apply where anymore, and can't seem to find it in the Knowledge Base.

Access restrictions prevents avatars from being physically present at any altitude below 50m AGL.

Object entry prevents prims that aren't being sat upon from entering a parcel, there's no height restriction there (and in wonderful Linden logic, coalesced prims that are being rezzed and span multiple parcels are exempt from object entry *and* no build as long as the avie can rez on one of the parcel).

Thanks Kitty. That's kinda what I remembered, with the extension that explicit "blacklist" bans also go all the way up. I didn't know that about coalesced objects--I wonder if it only applies when the Editor is open while rezzing, so the objects will start out Selected--although that may have nothing to do with it: I know the Editor can't move a prim into a no-entry parcel, and I seem to recall that it can't move one onto a full parcel, either, but I get both those errors when rezzing coalesced objects, too.

It's a bug and I'm not sure if it consistently works though, but you can get through.

What happens is that you have a coalesced item that you rez on your own land, but if you do that close enough to the border (so that the item who's name you see in the inventory can fit onto your land, but the rest of the - unlinked - coalesced objects don't necessarily) you can - accidently or purposefully - clutter up your neighbour's land despite it being "no build", "object entry on" or whatever else should normally keep you from being able to rez things on their land.