Yes, there are female minotaurs.

Nothing in the race description specifies that the race is male-only (the way satyrs are explicitly male-only and hamadryads are explicitly female-only), ergo there must be female minotaurs.

It's just a hypothesis, but based on the people I've known who've played minotaurs, the reason there are no images of female minotaurs is that very few people play them. Also, a quick Google image search for 'female minotaur' yielded a relatively low total number of images, and most of those were... fetish-oriented... rather than heroic. There aren't a lot of minotaur images in 4e books in the first place, and the artists probably opted for the traditional depiction.

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.