Im not sure if we have the same case.....
I got my MPNP nomination May 2, 2016 (but actually it was approved April 29) and valid until Oct. 29, 2016.
Since I need to have a new IELTS before I submit to CIC that time, I took the test in July. Then, with all the preparation of our documents and payments I only submit my documents and received by CIC in Oct 19, 2016. Imagine, I almost missed it.
My papers were processed bcoz its still valid. You mentioned that your expiration is April 2018. I think you can apply now to CIC/ IRCC.