1) We did a minat with my sisters that we will read an entire Quran together. In my mind my niyyat was to read together turn by turn like I read first then she reads. In her mind her niyyat was that we finish the Quran but we read one sipara each of us separately.
So I explained her when we start so we followed my niyyat but its really hard that sometimes we miss it
Is it possible to change the niyyat and follow hers?

2) And for the days that we missed , do we have to do something in addition to isteghfar?

1) Yes you can do that.

2) Nothing to be worried about that. But it’s not harmful if you ask Allah forgiveness in any situation. Seeking Allah’s forgiveness is always appreciated. May Allah forgive all of us.