Can a period of performance of a reimburseable MIPR extend past the funds availablity of obligations under any other circumstances other than a non-severable contract?
According to the DoD FMR, Volume 11A:
Because of previous instances of abuse of Economy Act orders, limitations on the use of Economy Act orders have been imposed. Economy Act orders may not be used by an agency to circumvent conditions and limitations imposed on the use of funds, including extending the period of availability of the cited funds.
It appears that unless it is non-severable, you cannot extend the period of the MIPR to cover a period beyond the true availability of the funds which in the question would be 1 Oct 2019 - 30 Sep 2020.
I would recommend seeking a legal opinion to confirm this result, but the DoD FMR seems very clear on this matter.