The answers are not going to be exact but
For midline I would add the 6 numbers and then divide by 6 to get the average.
The amplitude is decreasing over time so I don't think you can give it a set value.
The period is the time taken for the wave to start repeating itself.
the crests seem to be at about
4am on the 26th April, 4am on the 27th april, 4am on the 28th April. .... etc
So what do you think the period is?