My spouse and I are buying our first house and we're looking into opening a WAMU Mortgage Plus account. Our financial parameters are: (a) debt-free, (b) available liquid assets in an investment account, (c) good credit score.
The WAMU website and their additional fancy animated site showcase the account but don't really give a lot of detail on how things are going on, so I thought I'd ask the experts around here. 
Here's a scenario that I don't understand. Assume I make a $300K loan for 30 years at 6%. Then say I decide that for a while investments are not going to make much money so I move $300K from the investment account into the WAMU account. What will the monthly payment be? My understanding is that there will be no more interest to pay, so the montly payment will be 300K / 360 months = $833 dollars a month. Is that correct?
Now, say after a while I decide to transfer some money back from the mortgage plus account into the investment account. The websites do not make very clear how easy that transfer is, and what the conditions are. After I've made that $300K deposit, how much can I extract back later, how often, and how easily? Ideally I could take out most of my money whenever I want, but they don't quite say so in the scenarios they present. From what I could understand, as you add money into the account your credit line increases, but they don't disclose the formula they use to calculate that increase.
If anyone has more details on the workings of the WAMU mortgage plus account, I'd be grateful if you posted them here. I think it's quite an interesting account worth a new thread.
Edit: Just to avoid the "repost" stigma, I had read the other thread on this account, but it doesn't answer my questions above.
