Your RAW interpretation is correct
The interaction with lesser restoration
When casting lesser restoration on a target affected by feign death, the RAW result would be the same as casting it on a target affected by the likes of blindness/deafness, contagion, sunbeam, or any other spell imposing the blinded condition: the blindness is ended.
This is odd, though, because with feign death the target is in a state of blind and immobile catalepsy verging on death because they opted into it, not because they were harmed. My guess is that the designers didn't consider that anybody would bother to cast lesser restoration on the same target (or didn't care). The RAW leads to some narrative dissonance that I suspect isn't intended, which I'll address in a later section.
The interaction with freedom of movement
When casting freedom of movement on a target affected by feign death, there's no condition being ended. Rather, freedom of movement effectively suspends one of the effects of feign death.
Here's the relevant text from freedom of movement:
For the duration, ...spells and other magical effects [can't] reduce the target's speed...
And from feign death:
For the spell’s duration... [the target's] speed drops to 0.
In this case, we need to consult the rule for overlapping spell effects to figure out how they should interact (see Combining Magical Effects from the Spellcasting section of the Basic Rules):
The effects of different spells add together while the durations of those spells overlap.
So the RAW result is that the effects add together. But what does it mean to "add together" two directly contradictory effects? Since they're mutually exclusive, one spell has to supersede the other (or they have to cancel out, thereby imposing no movement adjustments on the target, which in this case is equivalent to freedom of movement's benefit anyway). So which one should the DM choose? Well, the entire point of freedom of movement is exactly to counteract effects like this!
Once again, by RAW, the result is that the effect of feign death should be superseded. Yet again the RAW leads to some narrative dissonance that I suspect isn't intended, so let's address that now.
How to resolve the dissonance
I have no evidence that the RAW result isn't intended. However, you're asking the question because, like me, it just feels like it shouldn't work this way. As you said in your question, "this doesn't quite seem like it's the intent of the spell." So let's operate under that assumption: that there's some dissonance to work through.
At my table, feign death would supersede the other spells, because my gut tells me that when you opt into its effects you don't get to have your cake and eat it too. At your table, you're free to use the RAW if you think it promotes more interesting gameplay or narrative (magical espionage action: "huh? what's that? it's just a body"), or you're free to share my gut feeling and declare that feign death supersedes. In any case, it's always up to the DM to run the game in a way that makes sense for them and their table, so you should do that.
My suggestion is to err on the side of the ruling that seems most consistent in your game's world and for the tone of the story you're trying to create. What works better for you: that a cataleptic creature indistinguishable from dead can see and move or that it can't? Your answer to that question will solve your problem.
No
The rule for swapping out eldritch invocations says you can swap one for another eldritch invocation. If you pick the same one again it is not "another".
However
You arguably could switch BoAS out for another eldritch invocation, and then pick it again on a later level up. There's no particular reason to believe the rituals that were already in the book disappear when you lose the feature, although you certainly couldn't scribe new rituals or cast existing ones while you don't have it, and the invocation doesn't mention that you can only gain the two 1st level rituals you get upon picking it once. This seems like a bad idea to me though, considering you can just scribe new rituals into the book for a fairly low cost as long as you can find them written down somewhere.
Best Answer
Whether a patron is fooled by the Feign Death spell or not is irrelevant. I would suspect not since we're talking a about low-level spell vs a god-like entity, but that's more in the realm of the DM's prerogative.
The important point to note is that the Feign Death spell says that the target only appears to be dead. They aren't actually dead, of course.
And even though this appearance can even 'trick' spells, it has no effect on the warlock's class features such as the book or talisman.
Both of these features state that they crumble when the warlock is dead: The warlock isn't dead. Hence by RAW they don't crumble.
So if someone knew that the target was a warlock, and knew that a warlock's special Book of Shadows or Talisman vanished upon death, then they would know that they weren't really dead, circumventing the purpose of the spell.
How common it is for someone to have this kind of knowledge would, again, be up to the DM. In my opinion it wouldn't be that common and would assume that anyone wanting to know this even knew the character was a warlock in the first place ("class" is a meta-property, not in in-universe mechanism).
The warlock could, of course, take pains to hide their book or talisman before feigning death!