Scaling score to parts

Normally, a "part" is just that - a single part for a single instrument or voice. So what you're doing here is a bit unorthodox, and it means some things that normally work as expected right out of the box might need to be adjusted for these special "duet" parts. Unless you are saying, it's meant to just be a single part that happens to have them sing at times? if so, definitely no need to use two staves. Just give them their vocal notes * lyrics on the same staff as their instrumental notes (I'm assuming you don't want them try to do both at once, but even that would be possible using multiple voices). Not sure what you mean in asking how to do it, notes intended to be sung are entered exactly the same as notes intended to be played. Whatever you did to enter the notes onto the vocal staff, just do the same thing but directly onto the instrumental staff. If maybe you mean, how to get the playback sound to change at that point, add an instrument change from the Text palette.

Normally, the staff names would not appear in parts, because the default is to hide instrument names when there is only one instrument. Looks like you turned that setting off in Format / Style / Score, so that's why you still see staff names even if you generate parts for only single instruments. but you get them for your special "double" parts because any time there is more than one instrument, it would be the norm to show names to differentiate them. If you do decide to keep these "double" parts (i recommend against, takes twice as much space for no good reason, and makes it harder to read on top of that), then just delete the instrument names.

Did you use a custom style file or instruments file or template in creating this score? That might have something to do with why that things that normally work as expected by default are not in this particular score. Or maybe you changed some settings manually? Understanding how the score got into this unusual state would help us show you how to fix it.

Beyond that:

• Your part names are at top left, on the first page (the title frame).
• If you want a title page only for the score, best to add it separately using a PDF editor; MuseScore doesn't support that directly.
• Can you be more specific about which elements you feel aren't scaling as expected? things look as I expect given how you set them up - 26pt for rehearsal marks in the score is huge but doesn't appear to because the staves are scaled so small (unreadably so, I would say - I'd avoid anything smaller than 1 mm for the staff space, Your parts are also unreadable smaller though - 1.2 mm might be OK for a conductor score, but it's not usable for the parts. Anyhow, the rehearsal makes seems correctly sized there, 26 pt but scale down as appropriate for that small a staff size.