Sending a letter back in April would have NO effect on anything, IRS hasnt been there to open any mail.

You can't amend for a bank account number, and again, nobody was opening the mail, so thats another lost cause.

If the bank account was incorrect, it gets bounced back to IRS and they issue a check to the address on the tax return.

Many returns filed in April are still stuck in processing....is that what you see when you check the Where's My Refund website?  If so, you'll want to call in, IRS does have people answering the phones now, but the wait time will be long, put it on speaker phone and keep yourself busy while you wait.