Is it safe to use the CBC-MAC as a Initial Vector and Key Check Value in deterministic encryption?
Am I thinking correctly and will an malicious user (which has access to encryption keys) not (or very hard) be able to generate a collision in my IV-ciphertext combination so that the server is grouping unequal messages together?

If the server checks all the bytes in the IV-ciphertext (and declares a collision only every byte for two encrypted messages are identical), and if you can decrypt it (and a decryption of an encrypted message will always output the original message), then it is obviously impossible to generate for two distinct messages to be declared as colliding (and you don't have to worry about the details of the encryption method except for these two facts).
Consider two distinct plaintext messages $M \ne M'$, they will be encrypted into ciphertext messages(which includes the IVs) $C = E(M)$ and $C' = E(M')$.  Now, the decryption process is a function of the ciphertext and the keys (and nothing else; that's why I counted the IVs as part of the ciphertext).  If we had $C = C'$, then what would this common ciphertext decrypt to?  If it is $M$, then we have a message $M'$ that, after encryption, could not be decrypted properly (hence violating the assumption that decryption of an encrypted value always gave back the original plaintext).
Hence, a malicious user cannot generate a message that the server would confuse with another message.