A showcase, demo or sprint review represents one and same thing. A sprint review is an informal meeting that typically takes the form of a demo of the completed features or what is accomplished during the sprint to the product owner.
This can be done as and when the work is completed over waiting for last day of the sprint to arrive. This mitigation the risk of incorporating review comments which may result in sprint failure if the demo is done at the last day of the sprint. In case if there are review comments, that should be fixed and once again demo to be conducted for the story which had review comments.
The whole team should participate. Although the team needs to get the buy in from PO to change the story to done-done while the feedback from whole team helps. You can invite other teams, support or customer as needed.
One of the core objectives of the demo is to review progress against the sprint goal and team commitment. It is an opportunity to get everyone on the same page in terms of what was accomplished, what’s still on progress and what tradeoffs were made.