How likely is it that all the addresses involved in a SINGLE multisig transaction input/output belong to the same wallet/owner?
Multisig is useful for both a single individual and for a group. Individuals can use it to secure their funds by using a 2/3 address and distributing the keys. Groups can use multisig to implement an escrow transaction for goods or services. 

These, and other use-cases, for multisig are being used today, but it's impossible to know the proportion of individual-use and group-use. There is no way to know for certain if they are controlled by the same person, unless you have more information than just what is available in the blockchain.