Using SCRUM in a team is a process, you won’t get it right after a few times but eventually, things are going to make sense. But it is not enough for one person to get what’s the point of such team organization. All team members must be aware of the benefits and weaknesses when doing SCRUM.
Here are two thoughts that I would like to highlight, based on my experience leading a fairly agile team with medium work interruptions.
When adding stories to the sprint, the team makes a commitment to finish them before the next sprint planning. Counting partially completed stories would break the sprint agreement and would not be an accurate representation of the team capacity (DoD is not met). This is important when assessing the next sprint capacity and since the velocity is made only of finished story SP, the team will commit more confidently.
The team should edit half-done stories and re-evaluate their SP and put them on the next sprint or on the backlog (depending on the story priority).
The placeholder story is there only for bookkeeping how much effort has gone into work that was initially not planned for the sprint. As such, the SP of the story should not be added to the velocity since the team didn’t commit to any effort.
It is, however, important to adjust the placeholder story points in order to plan for the known unknowns.