I do not think these games are designed to intentionally exclude anyone. Rather they are designed to attract gamers who inherently seek challenging games, for the sake of being challenged.
2) Do you think games have a moral obligation to offer modes accessible to disabled gamers?
Not anymore than a movie should have to provide a braille synopsis, or a novel should come with built in Cliff's Notes, or an album should have to provide an extensive spectrogram. There are many genres of gaming which can cater to someone with physical disability. It should not fall on a hardcore action game developer's shoulders to accommodate every possible player. Instead of the developer putting in a "Press A to Win" mode, rather the player can watch a Let's Play to experience content tourism.
3) Do you think games have a moral obligation to offer modes accessible to people not as skilled in games?
No. That makes about as much sense as abolishing the Special Olympics, because ethically the normal Olympics should just make its events more accessible.

4) Do you think the inclusion of the suggested variants (Skipping levels/bosses/infinite lifes, etc.) takes away from the artistic goals of games?
Yes.
5) Do you think the same applies to difficult movies (Eg. Baraka) or a difficult book (Don Quixote) or, say, Picasso's art? Is all art inherently exclusionary or ableist?
Not all art. Some art is meant to be easily digested potboiler. But some art is meant to challenge its beholder. If the art is purposely meant to challenge the beholder, that doesn't mean the artist wished to intentionally exclude the beholder. Rather the artist wants the beholder to experience the art on the same level its creator envisioned it to exist upon. To experience the art from a diluted or watered down standpoint, not only alters the intended perception, but also changes the art itself.
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A personal anecdote. I beat Demon's Souls for the first time this year. The game was hard as hell, especially since I wasn't using magic. There were times I didn't know if I could even beat it. I seriously questioned my own skill, and occasionally felt demoralized. However, I kept coming back. I was determined to "git gud". And eventually I did, eventually I beat Demon's Souls. The feeling of elation of having overcome such immense challenge lead to an emotional reward which was purely euphoric. If I had instead skipped the hard bosses, and just lackadaisically screwed around to the credits, I would not have felt that same feeling. I am absolutely sure FromSoftware inherently designed Demon's Souls to provide the experience I had. That was the point of Demon's Souls. If Demon's Souls had been optionally much easier, its point would have been lost. The artistic purpose itself would have been moot.