As for your first question yes, you only change scores based on the beast shape spell (mainly just size modifiers, you can look at the link in the comments to see answers on how that works).
As for your second question, yes, you gain all of the creatures natural attacks and adjust them to your modifiers.
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.
Best Answer
I say, Yes.
It's not always easy to tell whether a creature is using its Strength or Dexterity for its attacks. It's really up to the DM to decide how a creature fights and if a Strength drain affects its fighting ability.
Compare the creature's Challenge Rating on page 8 of the Monster Manual to find what its Proficiency Bonus is. From that we can compare it's Dexterity and Strength Modifiers to make a guess. Thankfully in the case of the Giant Scorpion, it looks pretty clean cut to me.
The Giant Scorpion has a Challenge Rating of 3. As per the Monster Manual (pg. 8), that gives it a Proficiency Bonus of +2.
It's Dexterity and Strength Modifiers are +1 and +2 respectively. To get its Attack Bonus of +4, it would probably have to use its Strength. Its damage dice are also modified by +2 which seems to lean towards a Strength based attack.
That's just my thoughts on this. Ultimately, it's up to the DM of your group to decide exactly which creatures use which stats to fight with as it's often not clear from the creature description.