As the title says, can UMD be used on an artifact to emulate a different plane? Specifically the Fellnight Queen's mirror.
By RAW, no. UMD is primarily about getting around class, ability score, alignment, or race restrictions with magic items. The other functions are Decipher a Spell and Activate Blindly. This definitely wouldn't fall under the former and, as written, doesn't really work with the latter either. Activating blindly is about convincing an item that would otherwise work right now if you knew how to use it to activate for you. Since the artifact in question here is one that apparently doesn't work on certain planes, activate blindly apply.
This WOULD make for an interesting house rule. However, if I were GMing that game, the DC would vary based on the power of the item, the duration you're going for, etc. Rest assured, it'd clear 30 at the very least because, while I'm all for skills letting non-magical characters do amazing things, artifacts are meant to be amazing in their own right and not things which bend their nature easily to some schmuck who doesn't know magical negative from magical positive.
I'm considering Skill Focus UMD, at 12th or 13th level, that should push me to +25+ish, so DC 30, 40 maybe 50 could be doable.
I believe the answer is no.
Let's look at what UMD can do:
Activate Blindly: Some magic items are activated by special words, thoughts, or actions. You can activate such an item as if you were using the activation word, thought, or action, even when you're not and even if you don't know it. You do have to perform some equivalent activity in order to make the check. That is, you must speak, wave the item around, or otherwise attempt to get it to activate. You get a +2 bonus on your Use Magic Device check if you've activated the item in question at least once before. If you fail by 9 or less, you can't activate the device. If you fail by 10 or more, you suffer a mishap. A mishap means that magical energy gets released but doesn't do what you wanted it to do. The default mishaps are that the item affects the wrong target or that uncontrolled magical energy is released, dealing 2d6 points of damage to you. This mishap is in addition to the chance for a mishap that you normally risk when you cast a spell from a scroll that you could not otherwise cast yourself.
Decipher a Written Spell: This usage works just like deciphering a written spell with the Spellcraft skill, except that the DC is 5 points higher. Deciphering a written spell requires 1 minute of concentration.
Emulate an Ability Score: To cast a spell from a scroll, you need a high score in the appropriate ability (Intelligence for wizard spells, Wisdom for divine spells, or Charisma for sorcerer or bard spells). Your effective ability score (appropriate to the class you're emulating when you try to cast the spell from the scroll) is your Use Magic Device check result minus 15. If you already have a high enough score in the appropriate ability, you don't need to make this check.
Emulate an Alignment: Some magic items have positive or negative effects based on the user's alignment. Use Magic Device lets you use these items as if you were of an alignment of your choice. You can emulate only one alignment at a time.
Emulate a Class Feature: Sometimes you need to use a class feature to activate a magic item. In this case, your effective level in the emulated class equals your Use Magic Device check result minus 20. This skill does not let you actually use the class feature of another class. It just lets you activate items as if you had that class feature. If the class whose feature you are emulating has an alignment requirement, you must meet it, either honestly or by emulating an appropriate alignment with a separate Use Magic Device check (see above).
Emulate a Race: Some magic items work only for members of certain races, or work better for members of those races. You can use such an item as if you were a member of a race of your choice. You can emulate only one race at a time.
Use a Scroll: Normally, to cast a spell from a scroll, you must have the scroll's spell on your class spell list. Use Magic Device allows you to use a scroll as if you had a particular spell on your class spell list. The DC is equal to 20 + the caster level of the spell you are trying to cast from the scroll. In addition, casting a spell from a scroll requires a minimum score (10 + spell level) in the appropriate ability. If you don't have a sufficient score in that ability, you must emulate the ability score with a separate Use Magic Device check.
None of those allow you to emulate being on a plane.
So simply, no. There does not appear to be anything UMD can do for you, no matter how high your UMD value is.
By RAW, no. But that's probably just because the core design team never considered such a thing. If you want to allow it as a DM, I don't see a problem with it in general. You will need to make up your own DCs. I would think it would be quite high.
A person with a high UMD should be more effective at using a magic item than a person who is just using a magic item. Since person A can possibly activate magic items abilities that are unavailable to person B, who is just using said item at whatever abilities it has. Naturally, this only applies to those magic items that have different levels of power.
If the artifact is only magical while on that plane, then no amount of UMD will help. If it is always magical but can only be activated while on that plane, that is something different and I can see a precedence for UMD. It could be, however, that the artifact's description lacks that kind of detail.
A person with a high UMD should be more effective at using a magic item than a person who is just using a magic item. Since person A can possibly activate magic items abilities that are unavailable to person B, who is just using said item at whatever abilities it has. Naturally, this only applies to those magic items that have different levels of power.
No, UMD lets you use a magic device as if you were a competently trained person who could use the device. It doesn't let your supercharge items or use them in a way that a properly trained person could not.
If a person can activate an item, they can general activate all abilities of the item (I say generally because some abilities are contigent upon certain things, but it is not an issue of activation).
You are essentially asking a question like, can a non-Paladin use a Holy Avenger?
Yes, they can but only as a +2 cold iron longsword.
Using UMD, it's a little ambiguous about whether UMD can trick the item into believing you're a paladin. It may be covered by the "emulate class feature" ability of UMD, but is unclear. Note you would also need to emulate the Lawful Good alignment, per UMD.
If you were successful (assuming UMD can be used to emulate being a paladin) then you would unlock it's other abilities. It would function as a +5 holy cold iron longsword with the ability to perform Greater Dispel Magic and give spell resistance. Exactly the same thing a paladin gets.
That's what UMD does. It doesn't let you use it better than the paladin, it would let you use it as if you were a paladin.