Is it possible to add fields to a table that uses partitioning and already has data?

Question

MySQL

The essence of the question is this.
For example, I created a table:

CREATE TABLE tm1 (
s1 CHAR(32)
s2 CHAR(32)
s3 CHAR(32)
placename(32)
) PARTITION BY KEY(placename) PARTITIONS 100;

Filled the table with data. Can I add more columns to the table after that? This is the first.
Second: The table is planned to have several million records. Every 100-200 thousand records will have one placename. Is it appropriate to partition the table by the placename key for these purposes?
Third: Can I, after creating the table, change the number of partitions, i.e. that in an example at creation PARTITIONS 100 is specified?
Fourth: Will it be much faster to process a query for a selection with conditions from a table in which there are about 200 thousand records than from a table in which there are about 5 million records, but partitioning is done, which breaks the entire table into pieces, in which, again, about 200 thousand records?
I have not contacted partitioning before, so I ask for your help in this matter.