Missionary accounts - closing, transfers

What are the requirements (i.e. balance = 0, X many months have passed since missionary returned, etc.) for a missionary's account to close? We have a family who has Daughter A returning from her mission in February and another Daughter B who may leave in July. If there is excess in Daughter A's account, can we leave the money in her account until July and then transfer to Daughter B? What is the cutoff for when that is no longer possible?

As far as I know, as long as there is a balance in the account it will remain active. However, it's possible that the stake might query you about why the account hasn't been cleared.

Bottom line up front: Don't worry about it. All donations are the property of the Church, and there are no individual "accounts" as such. When the last payment for the first missionary is swept about one month prior to the end of her mission, you can move any remaining funds from her "account" to the ward missionary fund, where they can wait until the second missionary enters the MTC.
Worst case scenario: the stake may ask why you have unallocated funds hanging around in your ward missionary fund. Just tell them why, and they will get off your back. The general rule is that you can keep up to 90 days of mission payments at the ward level. Any "excess" funds not needed for the coming 90 days should be transferred to the stake missionary fund. Families are discouraged from donating missionary funds until they are needed by the ward.

Hello!
My bishop has asked me (the ward clerk) to close out a recent returned missionary's account and transfer the remaining funds to her brother's account, who is currently out in the mission field. My question is how does one do that? Is the same procedure the same for transferring funds from the "Other" account to a specific account? Thanks!

In MLS, select Finance > Enter Transfers from the menu and then select the appropriate accounts and enter the amount left in the missionary's account. It's pretty much the same thing as for transferring between other accounts except once you select a ward missionary account on the left side then you can only select from ward missionary accounts on the right.