You can only pay the whole thing off there's no option to lower it after you have received the phone.

Can I put that same lump sum towards my account instead and I would just have a “credit” in my account for like 3 months?

I’m not sure what you mean by getting a credit, but I don’t think that is how it works. Either you keep making your monthly payments to your Tab until it is done or you pay it off in one go. You do not get any credits for doing that. 

Sorry...I just mean if I pay my normal cell phone bill with a lump sum payment, it would essentially work out the same as if I put it on my tab.

It could work out that way, but it is worth noting that you are still on the hook for the remaining Tab Bonus if you choose to cancel, change, or upgrade your plan before the 24-month period ends. You can visit this link for more information. https://www.koodomobile.com/en/help/Tab-bonus

Yes you could do that. Any money you overpay on your account will just sit there and be carried over to your next bill.