In the first instance, with Mix Master, the ownership of the I.P. (the songwriting) is yours alone as a mix is not the song (words+music). The Mix & Master can be owned by the record label. The Mix & Master should always be correctly credited to the person who did the work - incl if someone comes and does some extra work. None of that suggests further payments or inability to use the music for any reasonable purpose.
In the second instance, DO NOT assume that Creative Input is work for hire as many do. Even Fiverr make it clear this is not always the case.
The difference is this: If you have someone else simply perform your line to eg replace your guitar line with their performance, this does not mean that you owe royalties on performance as you simply hired them to perform a task like digging a hole, only with a guitar.
If you have someone add their creative flair, esp if you have them create the Riff that the guitar plays and that adds significant value to the final song, then they can be considered for a composer credit.
- You give Keith Richards a Chord Progression and ask him to create a guitar part: notes, timings etc., he would ask for credit seeing he did more than just play what was there seeing he added the unique musical construction that makes your song work.
- Sting wanders into the studio where Dire Straits are working on a song. Stings starts to croon “I want my MTV”. Mark K is chuffed as it really makes the song fire (and into a Hit) so Sting gets a big songwriting credit.
Bottom line, it is best that you clarify these datils with the person that you work with. An experienced professional already knows about these things and may well let you know up front or be happy to guide you.
The thing I will advise against is getting up-tight about it seeing it breaks trust. Better to have a hit that you get 50% of the $millions than a bust in which you don’t give away any credit points.