What is the best choose for PK in Partitioned Table?
For the first question, there's no issue with using the same function for two tables as long as the definition of the partition function doesn't ever need to change. You mentioned deleting daily data and your partition function is planned to contain 32 partitions, so I assume that you're creating one partition per day of the month. If so, I can't see a reason why you would need to merge or split partitions. With that said, given that you just have two tables involved there isn't anything wrong with creating two separate functions as well.
Whether or not the two tables should exist on the same database files depends on how you're defining your files and filegroups. Given that you're only keeping 14 days of data and you purge the rest, I assume that you aren't putting some partitions on different classes of storage. 5 million rows in the Sens table really isn't a large number, so it's likely true that there will be no issue with using the same database files with no tables. This is just a guess based on incomplete information.
For the second question, evaluate whether or not a partition-aligned index is the right fit for the table:

An index that is built on the same partition scheme as its
corresponding table. When a table and its indexes are in alignment,
SQL Server can switch partitions quickly and efficiently while
maintaining the partition structure of both the table and its indexes.
An index does not have to participate in the same named partition
function to be aligned with its base table. However, the partition
function of the index and the base table must be essentially the same,
in that:

The arguments of the partition functions have the same data type.
They define the same number of partitions.
They define the same boundary values for partitions.


My own opinion is that you shouldn't partition a table with 5 million rows unless you have a very good reason. Partitioning is a feature designed for large tables. Why do you need to partition a table with 5 million rows?
For the final question, I have worked with partitioned tables that have computed partition columns. I do not recommend it unless you have no other choice. We consistently ran into odd issues including partition elimination not working as expected.