There is some disagreement on the Wizards' forum on this as well.
There is a ruling from customer service in 2008.
You must be 3 squares AWAY from the
square in which you started your turn.
If you end your turn only two squares
away from where you started you will
not gain concealment with Shadow Walk.
A ruling from 2009 contradicts that.
In order for the Shadow walk to
trigger the warlock needs to move at
least 3 squares away from the starting
square. He does not need to finish his
move 3 or more squares away from the
starting square
Incidentally the PHB3 glossary explicitly defines movement as
Whenever a creature, an object, or an
effect leaves a square to enter
another, it is moving, whether that
move is done willingly or is forced.
This means shifting, teleporting, and
being pushed are all moves, for
example.
So if you were to teleport 3 squares on your turn (through a non-immediate power though please!) you should trigger Shadow Walk.
Primary Consideration
Due to the controversial nature that this specific problem represents, it takes a mature player and a mature DM to handle this specific scenario well. Ensure that the player and DM have a conversation about something of the magnitude of power revocation BEFORE it is implemented in play...
If you are the DM and this is established as part of your world, ensure that players know this up front if they are considering playing a warlock (or any other class likely to have powers revoked for any reason).
If you are a player and you want this to either be highly probable, talk with your DM. It can create an intriguing story.
Already covered
RAW, there isn't any explicit text that covers a warlock's powers being stripped. Also, there is no class that has rules text covering a loss of powers, save the Paladin whose powers change form when he becomes an Oathbreaker. (DMG p. 97)
Lore from stories associated with various pact-style magic demonstrates and sets precedent that it could be a good story hook, allowing for a very interesting story line that leads to all the things that were mentioned in the original question.
Also Consider
The power belongs to the entity to dole out. If they granted it, it is likely true that they could take it away. This is more of a reference to old literature, to what makes sense, and to what would be fun with the game you and your DM seem to be trying to build.
The rules do not state the specific pact, this is the part that cannot be stressed enough. The power that they currently have should mostly be considered as payment for services rendered. If they are not completely paid for (ongoing payment, for example), then try to avoid stripping a lot of their power from them. Make it minimal, but noticeable. Lower spell slot levels by one levels as that part hasn't been paid in full, if that helps the story line, but do not completely cripple the character.
As for the specific scenario posted in the question:
The devil is highly unlikely to strip power from someone if it is part of a contract. To do so is highly unorthodox for a devil, and would be looked at even more poorly than an upstart servant that the devil couldn't control. The devils live by their contracts, and although adding loop-holes may be favored, rescinding an agreement is not. If a devil were to rescind the power of one of their warlocks, it is likely that another entity would try to mock them by taking the contract instead. This is even more true of a relatively powerful warlock. That said, a devil is also very likely to add a hidden clause that prevents the warlock from using their powers against the devil, with revocation of powers being either temporary or permanent, depending on the devil, the warlock, and the devil's disposition at the time.
They will follow the letter of the contract completely, but don't care about the spirit of the contract. If the devil is powerful enough to grant powers as a patron, they are likely intelligent enough to be more careful about the contract, though a particularly savvy and/or intelligent character could manipulate them in to a contract that is written to the benefit of the character more than the patron (protecting themselves from abandoning the patron, for example).
If you really want to do it
The best way to add the possibility of power revocation for story-line purposes is to ensure that it is an active part of the party's story (as opposed to a passive one). Allow the party to try to stop the powers from getting revoked, or have another patron (possibly a more sinister one) inform the warlock that they are going to lose their powers and offer to be a surrogate patron.
Do something that directly involves the warlock in whether they lose or retain their powers.
In short
Is it possible? Rules don't state that it is, but this is something a DM could easily say yes to with good justification to back it up based on game world.
Should the DM take this path? Probably not, or at least not seriously. If he does, then he should offer a work-around. Don't strip power from a character/player without offering a way of obtaining it again. Don't make the game less fun.
Best Answer
The Prince of Frost
One option is the Prince of Frost, detailed in Dragon Magazine 374 and discussed here on Eberron author Keith Baker's blog. As I understand it the Dragon article was written to fit in any D&D setting, but it was written by Keith Baker with the specific themes of Eberron and Thelanis in mind.
This Fey could be a suitable, if somewhat villainous, patron for a Valenar elf. The Prince's story is that of a scorned lover - he awaits the reincarnation of the mortal he loved and the warrior his beloved spurned him for, and spends the rest of his time tormenting mortal heroes out of spite.
That fits with one angle on the Valenar - their culture is focused on emulating the ancestors through conflict, so finding mortal heroes and making life difficult for them isn't exactly out of your area of interest. It might make for some tension with the rest of the PCs, and you may have to prioritise your goals against the Prince's; depending on what exactly you and the other players want out of the game that could be a positive or a negative!
Other options
There are some alternatives that don't answer the question you asked (and which I wouldn't post without having led with a published Fey that might be suitable), but which might be useful to you.
Adapt a real-world fairy story
Keith Baker also writes here about ways to adapt real-world fairy tales as Fey in Eberron, using the example of the Sleeping Prince (based on the Sleeping Beauty story).
For a more Valenar angle on that, you could take the recurring figure of the Woodcutter or the Hunter, as seen in Red Riding Hood, Snow White, and the like - an outsider figure set apart from more civilised folk, who does the dirty work of killing that others might scorn, but who can be swayed (as in the Snow White story) by appeals to whatever kind of virtue they value, would work very well as the patron for a PC. Such a fey could easily find common ground with the Valenar.
The Hexblade patron
Some of your reasons for considering an Archfey Pact might also support the Hexblade from Xanathar's Guide to Everything - drawing power from a legendary cursed weapon, rather than a lord of the Fey, would also fit well with Valenar themes.
As a culture that focuses on the stories and deeds of heroic warrior ancestors, it makes sense that legendary weapons might come up in some of those tales. The earliest ancestors were a slave insurgency against the Giants in Xen'drik, so the flavour focus on Shadow does also fit - stealthy violence is very much a part of Valenar culture.
There's also a good hook in the Valenar section in Rising from the Last War:
That opens up further options; whatever cursed blade you've pacted to could be an old Tairnadal Elf relic, perhaps one lost in one of these magical anomalies, but could as easily have roots in the goblin cultures the elves fought - the Dhakaani Empire were some of Eberron's most gifted creators of magical weapons, and claiming power from a trophy claimed from a foe is, again, very much in line with the Valenar focus on warrior-heroism and their roots in a rebellion.
The other reason I mention the Hexblade is the mechanics - medium armour proficiency, shield use, and the option to use Charisma for attack and damage rolls fits nicely with the idea of your character as someone who's emulating a warrior ancestor, but using their pact magic (and high Charisma score) to make up for lacking the aptitudes their ancestor's followers would usually rely on.