I answer both your questions below:
Your first question:
If I am reading the text correctly he can create the illusion of a
stone wall that's 10 feet thick, 10 feet high and 70 feet long.
Assuming all participants fail their Willpower saving throws, does
this illusory stone wall grant both groups total concealment?
My answer: yes
The description of the spells, which is an Illusion (figment), says:
This spell creates the visual illusion of an object, creature, or
force, as visualized by you. The illusion does not create sound,
smell, texture, or temperature. You can move the image within the
limits of the size of the effect.
The SRD also has the following to say:
Saving Throws and Illusions (Disbelief)
Creatures encountering an illusion usually do not receive saving
throws to recognize it as illusory until they study it carefully or
interact with it in some fashion.
A successful saving throw against an illusion reveals it to be false,
but a figment or phantasm remains as a translucent outline.
A failed saving throw indicates that a character fails to notice
something is amiss. a character faced with proof that an illusion
isn't real needs no saving throw. If any viewer successfully
disbelieves an illusion and communicates this fact to others, each
such viewer gains a saving throw with a +4 bonus.
So the answer is yes it provides total concealment for anyone failing their save: they cannot see through the wall. The illusion has no "facing" it appears as the object from all directions.
The caster has absolute proof it isn't real (they cast it) and so needs no saving throw. The rest do not receive saving throws until given a good reason to think it is not real or study it closely. Their sight tells them the caster has conjured a wall.
Some thoughts on this for the DM (who is always, has to be, the final arbiter on all of this, at least once the group has agreed the base understanding):
- The caster's party might have good reason to disbelieve if it is a common tactic of theirs, but this still requires a save, perhaps at +4.
- Someone recognising the spell cast as an illusion from a high arcana skill roll would have good reason. Just the fact it magically appeared out of nowhere is not reason enough, that's what magic does.
- Arrows flying or a fighter charging through the wall, for instance, may well give everyone reason to disbelieve even though their interaction with the illusion is due to someone else's actions.
- Even being given reason to believe that the wall is an illusion only grants a save. If the save fails then the character still sees it. They need "proof" it is not real to not require a save. What constitutes proof is up to the DM as, for instance, someone with no arcana skill may not have the knowledge to have it easily proved until they have their arm pushed through it.
- Re-saves should only be allowed (in my opinion) when something significantly new occurs. If an arrow flying through triggers a failed save then more arrows won't allow another save as arrows passing through has become a "real" characteristic of the wall for the character. However someone then running through the wall would be new and would trigger a save. It is up to the DM to judge when a new save is triggered.
And lastly even those who save see the wall "as a translucent outline" as it is a figment, though this will not provide any concealment.
Your second question:
If the wizard decides instead to create the illusion of a one-way
mirror with the same dimensions would that grant only the PCs total
The srd says the following about figments:
Figment: A figment spell creates a false sensation. Those who perceive
the figment perceive the same thing, not their own slightly different
versions of the figment. It is not a personalized mental impression.
Figments cannot make something seem to be something else. A figment
that includes audible effects cannot duplicate intelligible speech
unless the spell description specifically says it can. If intelligible
speech is possible, it must be in a language you can speak. If you try
to duplicate a language you cannot speak, the figment produces
gibberish. Likewise, you cannot make a visual copy of something unless
you know what it looks like (or copy another sense exactly unless you
have experienced it).
The key points here is "you cannot make a visual copy of something unless you know what it looks like", though you may be surprised where this leads us in ascertaining if you can make an illusion of a one-way mirror.
The illusion is not a mirror, it is a copy of a mirror showing what the caster thinks it should. A mirror would only work, only show a true depiction of the reflection, if the caster has a clear view of what is happening so as to be able to change the illusion to match the scene.
The see through side needs thought as, still, it is NOT a mirror, it is a copy of a mirror. The see though side will only show what the caster thinks it should. The caster would therefore need a clear view of the scene on the other side. Of course they automatically succeed on the save as they have proof it is an illusion so they can see right through it and can satisfy this requirement, if the lighting is good which it would need to be for any of this to work.
So my answer: yes
...if the lighting is good enough for any of it to work. However it is a complex illusion and I would suggest that the opportunities for disbelief triggering a save would be many as, for instance, watching yourself dancing around in front of the "mirror" pulling silly faces might well be deemed "close examination".