Couple questions for people with experience e-filing the 9465 with the 1040. 

I filed a 9465 with a clients return on April 17. The total due is 27k. 

Client wanted to pay 20k upfront and the rest over 4-5 months. I have no experience filing these electronically so stupid me I put the 20k as the initial payment on the 9465 instead of doing that separately using direct pay or something and just having the initial payment be the same amount as the other monthly payment. 

I do not want to have him rack up interest on this balance unnecessarily, but I also do not want a voluntary payment made outside the agreement to conflict with the initial payment I put on the form. In other words, I do not want the IRS to withdraw another 20k when the agreement is processed which would result in him overpaying 13k (as well as seriously messing with his cash flow). 

So my questions are: 

1 If I include the 9465 with the return which was filed timely, does the initial payment get credited as paid when the return was filed, such that he incurs interest only on 7k? Or does it get credited when the agreement is processed?

2 If the answer to the above is the latter, if I make the voluntary payment now to avoid accrual of any future interest, will the IRS still process the agreement as filed or will they disregard the initial 20k and just move on to withdrawing the monthly payments as normal? 

I know there might not be a real answer to these, I was more hoping if anyone had experience with this scenario and could share the results. 

thanks in advance.