Looks like MO will take your refund and use it to reduce the balance due on your payment plan.

Refunds, credits, or debt offset payments to an account will not be considered an IIA payment. However, this will reduce the balance due on an account.  https://dors.mo.gov/DOR/CACSGIIA/IIA/pdIIAPersonalTax.jsp

Probably. 

Missouri can take your refund for the 2019 tax return and apply it as a payment to your 2018 Tax return even if you have an installment agreement in place.

Additional information on the collection procedures for Missouri can be found in the following link.

Missouri collections