>whether to disregard door casings or other trim that's in place or to subtract them from the floor plan dimensions?

If you have one door will be much different than if you have a double door or each wall. Use common sense.

>How do you account for the amount extra needed for mitered corners? Doesn't that add about an inch per corner for a 1/2" thick baseboard?

Yes, plus saw kerfs. I'd suggest doing coped molding on inside corners instead of miters. You will also need to plan for which side is coped and which is not. Normally, you want the cope to be least visible from the room entry or common sitting areas.

>How much extra do you include for waste or errors?
Ha! How many errors are you going to have? Do you have only long walls, or a bunch of short walls?

>Do you make a list of cuts and figure that way? What's the best way to assemble the cuts into board lengths?

Yes. While you might come up with, say, 75 feet as a requirement, you don't want to have one wall made up of 1-2'
pieces all spliced together. Make a cut list and lay it out on your board lengths. Use a scale drawing. If you have difficulties with mental spacial balancing, you can use paper cut outs or if you have "cut list" software, use that.

>I'll be getting 16 foot boards. Anything else to consider when measuring and ordering?
Do you have a way to transport these home? Be sure to tie down front and back and not just in the middle if you are using a roof rack.

The link below showed up in my inbox this week. While it's for chair rail, much of the same you can apply to base molding. Part 1 is available, too, but mostly tool set up.

Here is a link that might be useful: chair rail project

"whether to disregard door casings or other trim that's in place or to subtract them from the floor plan dimensions?"

You are way over thinking this.

Order based on the floor dimensions, than add in at least 10% for waste (more if you are not an experienced trim carpenter, 10% for a good trim carpenter).

If yo have a lot of the same molding in multiple rooms you can cut some of them back to 5%.

Just remember that if you run short you get to pay the trim carpenter for returning to the job to finish (time is not free).

The doors and casing are normally ignored.
At least a few pieces will have defects.
If the piece are not long enough you need length for scarf joints.

Measure each section of wall over 6", round up to the next full foot @ each one; then add another 3%; you won't come up short.
Casey