First, some definitions.
A public key encryption scheme (PKE) is a scheme with public and private keys, where we can encrypt a message using the public key and decrypt using the private key.
A key encapsulation method (KEM) is a scheme with public and private keys, where we can use the public key to create a ciphertext (encapsulation) containing a randomly chosen symmetric key. We can decrypt the ciphertext using the private key.
A key exchange protocol (KEX) is a protocol that allows Alice and Bob to agree on a shared symmetric key. A two-move KEX is a protocol where Alice sends a single message to Bob, Bob replies with a single message to Alice, and afterwards they both have the same symmetric key.
Next, some context.
Any PKE with sufficiently large plaintext space can be trivially turned into a KEM.
Any KEM can be turned into a PKE by adding some symmetric encryption. (This is often called hybrid encryption.)
Any CPA-secure KEM can be turned into a passively secure KEX. (Alice runs KEX key generation and sends the public key to Bob. Bob creates a ciphertext containing a randomly chosen symmetric key and sends it back to Alice. Alice decrypts the ciphertext and gets the symmetric key.)
Some passively secure two-move KEX can be turned into a CPA-secure KEM. (The process that leads to Alice's message is key generation. The public key is Alice's message. Alice's state after sending the message is the private key. The process that leads to Bob's message and the shared symmetric key (in response to Alice's message, that is, the public key) is key encapsulation, and Bob's message is the public key.)
A passively secure two-move KEX can be turned into an actively secure three-move KEX using digital signatures. (Bob signs both messages and sends the signature as part of his message. Alice signs everything and sends the signature as the third message.)
In other words, these notions are tightly related. (There are lots of details and simplifications in the above, but morally, it should be roughly correct.)
Now to your actual questions (which are a bit unclear, but let's make an attempt at answers).
Compared to traditional PKEs like RSA-OAEP, designing a KEM and proving it secure is often easier. Likewise, compared to traditional key exchange schemes, designing a KEM and proving it secure is often easier. Then we can turn the KEM into a PKE using hybrid encryption, and we can turn it into a KEX with a standard conversion theorem. This is why KEMs are attractive objects to design and study.
Your sketch seems fundamentally similar to the above two-move KEX => KEM argument. However, that doesn't mean that a KEX is easier to design and analyse (from scratch). Experience suggests otherwise.
The traditional definitions of PKE, KEM and KEX doesn't allow the process to fail in the usual case. For traditional schemes, this has always been easy to guarantee. For newer schemes, this does not seem to be the case. Typically, there's some probability that things will go wrong (decryption failures). This makes analysis much more difficult, but not impossible (as NTRU shows us). Still, I don't think the presence of decryption failures changes the fact that KEM design and analysis seems to be easier than KEX design and analysis.
