As I understand, there are bilateral interline agreements and mult-lateral interline agreements, which are entirely different from code-share agreements.
Any airline that's a signatory to MITA will/should interline with anyother airline that's also a signatory to MITA. An airline does not have to sign agreements with every airline it wishes to interline with. Signing on to MITA automatically results in interlining agreement with all MITA signatories.

Strictly speaking, if you have separate tickets, MITA does not apply, unless the two tickets have been conjuncted.

I would like to hear from those with more knowledge of rules than I have.