Yes it does
Yes, you can add beast forms to your Wild Shape repertoire of beasts seen by casting conjure animals. The spell explicitly says the conjured creatures literally are beasts, and a creature of the type beast is all Wild Shape needs you to see in order to add its form to your repertoire.
Though someone might by tempted to quibble that they're not real beasts because they're really fey spirits clothed in the shape of beasts, look at it this way: First the Druid conjures a bunch of spirits that take on the form and behaviour of the real beasts they look like. The creatures so conjured proceed to behave and look exactly like the real thing. What better opportunity is there for a shapechanger to learn a new form to wear, than to see a nature spirit do the exact same thing as they're wanting to learn to do?
But it's of limited use
What you might actually run afoul of is limitations on metagaming, or a DM whose setting doesn't include just any creature you can think of (even if it's in the Monster Manual).
- A DM who wants to limit metagaming would be within their rights to ask you to justify your choice. If you try to summon a Giant Owl but your druid has never seen one before, a DM could easily say "How do you know those exist? You've never seen one. How is your druid "choosing" an animal (s)he has never seen or heard of before?"
- A DM who has developed a custom setting isn't straightjacketed by the Monster Manual and doesn't have to include everything in it. Giant Owls might not exist in a DM's campaign.
This also means that this is much less of a loophole than it might seem — after all, how big is the difference between the set of creatures a druid has personally seen and the set of creatures the druid knows about well enough to deliberately try to conjure? I can't imagine it's a very large number.
But in principle, yes, a conjured animal inhabited by a fey spirit is plenty to learn that form from. Just mind that, if you can conjure a particular animal, it's highly likely your druid has already seen it anyway. This makes this method of acquiring new forms both limited in utility and not much of a loophole to worry about, either way.
Yes you can
The rules for wildshape clearly state that you retain any benefits of features from your race if the new form is capable of using them. Since the Relentless Endurance feature does not implicitly or explicitly require any particular body parts (hands, for example) I see no reason why it should not be retained.
The rule on reverting says you automatically revert if you hit 0 hit points. Since your Relentless Endurance allows you to drop to 1 hp instead of dropping to 0, you never actually drop to zero and thus never revert.
... and there is no carry-over damage
The rule on carry-over damage says:
if you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form
Since you did not revert or drop to 0 hit points, this rule does not apply.
Best Answer
Here's the text of the Gnoll's ability (as found on page 317 of the pdf SRD):
And Wild Shape says:
In order for the druid to revert out of wild shape, they have to drop to 0 hit points. If they do so as a result of the gnoll's attack, it clearly did drop a creature to 0 hit points, and should be able to use Rampage. The fact that the creature suddenly turns into a different one that has more than 0 hit points doesn't matter.