you have a couple of options and I'm not sure which is the best for your situation.
1. Setup each account in Outlook and on the phone as separate accounts (and stop collecting your pop mail using outlook.com). Replies will always be from the correct account. If you need to share calendar and contacts with the phone, use outlook.com as the go-between. You can create a Microsoft account (formerly known as LiveID) using one of your email addresses and use it to with an EAS account to sync calendar and contacts.
2. If you only want to reply to mail using the work account, create the liveid for the work address - pop the accounts into this mailbox instead of the outlook.com address. This is basically what you are doing now, but the main address will be your work address.
3. Same configuration as now but only the outlook.com accounts gets mail - only the SMYP accounts in outlook are used for sending mail. Use this macro: Reply using the address a message was sent to - Slipstick Systems for sending messages from the address the message was sent to (it's not perfect - it won't work well when a message is sent to multiple people- you'll need to select the sending account in those cases.)