Almost any creature could escape before suffocating
In most circumstances, the victim would have ample time to chisel their way out.
Wall of Stone creates 10 foot panels (or larger). So let’s assume: (1) your spellcaster trapped their victim in the minimum size, 10' cube, and (2) they concentrated on the spell for 10 minutes, making the stone permanent.
I’m not aware of any rules that would cover suffocation in these conditions, so let’s fall-back on real-world facts. This Friday Fiction Facts: Trapped in an airtight room! article calculates it would take hours]for a person to suffocate in those conditions
“A moderately active or stressed person” would have about 17 hours until they experience symptoms such as “panting, dizziness, severe headache, vision disturbances” at which point we will assume they can not longer effectively chisel at the wall.
The wall is an object made of stone that can be damaged and thus
breached. Each panel has AC 15 and 30 hit points per inch of
thickness.
At 30 HP/inch times 6 inches, the victim needs to do 180 HP of damage to the AC 15 wall to escape.
Let’s assume our victim is a human commoner with no strength bonus. We’ll even take away their club listed in their stat block, and just give them an improvised weapons, with which they get no attack bonus. Each round, they can do 1d4 points of damage to the wall if they “hit” with a roll of 15 or higher.
With those assumptions, the commoner’s average damage-per-round would be 0.75 HP. On average (and with this many “rolls,” most attempts would be very close to average) it would take 240 rounds to chisel out.
A round is 6 seconds; there are 600 rounds in an hour, so our mild-mannered commoner can chisel free in 0.4 hours, or 24 minutes.
- (240 rounds) / (600 rounds per hour) = 0.4 hours = 24 minutes for a commoner to escape
So even if we make assumptions that our victim needs to rest three-quarters of the time they still get out with hours to spare.
If you are trapping an armed and dangerous creature, they are likely to be able to escape in even less time.
Edge cases
It’s easy to come up with edge cases where someone would not be able to dig out, or if the wall were made double thick, or more (which would require a lot of time since you have to concentrate for 10 minutes for each effect). In these cases the rules pretty much silent (the Portable Hole mentions death by suffocation in an enclosed space, but that’s a pretty different case).
Would there be tiny cracks in the wall, or the floor beneath it that allow air to seep in (like there are in the building where you sleep)? I think that is simply up to the DM. This is a world where you can breathe miles deep in the Underdark — the whole “how do we breathe” issue gets a little glossed over.
You can teleport from one place within the warded area to another within the warded area
Since, as you point out, the text of the spell only says you can't teleport from outside to inside, nothing stops you RAW from teleporting from one place within the warded area to another place within the warded area.
Just quoting again from forbiddance (PHB, pg. 243):
For the duration, creatures can't teleport into the area or use portals, such as those created by the gate spell, to enter the area.
I'd also say that teleporting whilst already within the area would also be RAI (Rules as Intended) since the intention of the spell seems to be to keep people out of the area, not to annoy those already within the area.
Best Answer
Yes, you can spread the area out over non-regular interior spaces.
The thing that immediately draws my attention about Forbiddance is that it doesn't specify a radius. It says it affects "floor space." That form of measurement in D&D is used exclusively or almost exclusively to specify the interior of buildings, bounded spaces, and containers; giving a strong indication that the spell is intended to work inside the walls of a building regardless of shape. The explanation for why multiple floors can be covered continues below.
No, you probably can't cover a non-contiguous area.
The RAW doesn't definitively indicate the floor space must be continuous or contiguous -- or, for that matter, anywhere near you, except for one section which you touch when casting. Overall, that's pretty weird because other spells like Guards and Wards and the various Wall spells do. Conversely, spells that definitely do target multiple non-contiguous areas (like Meteor Swarm) also give their area placement rules fairly clearly. It seems like neither position can be taken as assumed.
The only clues here is that area is specified as a single block, and the casting time is more than sufficient to tour any regular polygon with that area. Given those, the 'local and continuous' interpretation is by far the most easily supported. Even read that way, however, you could use a narrow "pathway" to connect the two buildings so long as the 40,000 sqft area was sufficient. (How narrow? 5ft? 1ft? 1in? ...Ask your DM.)
Yes, you can cover multiple floors, so long as they're connected by "floor."
In any case, changes in elevation absolutely can be contiguous (and usually are), and the spell seems to support this feature. Casting Forbiddence in a building on a hill would by RAW and should logically cover the entire floor area, even if it's not level. For the same reason, any floors connected by flooring (like staircases) can simply have the contiguous space spread upwards over them and onto the next floor.
The caveat here is that technically, one could call a staircase non-continuous flooring, since you can't actually walk on the rise of a step. Of course, there's a solution here too: access ramps. (For magic.)