Yegheettau39
Yes, April 1st is included, and the ticket is valid in Switzerland until Chiasso (border station).
A seat reservation is always a separate additional thing on top of the ticket, regardless of the ticket type.
You buy it e.g. like a regular ticket for a specific service, it appears as ‘seat reservation only’ in the suggested ticket options. Note that the train will reverse direction in Zurich.
I think I remember some other discussion here that you might try reserving for Basel - Lugano instead if no seat reservation option is appearing for Chiasso. Could be related to the sometimes very long waiting time of the EC in Lugano and that you may reach Chiasso earlier by hopping on a regional train leaving Lugano earlier than the EC.