1. Don't bash Steam support, its great if you have a head on your shoulders and know how to use it. For things other than eating and licking... ice cream
2. If you forgot your username and requested an email and it never arrived, get in contact with steam using any email that receives emails from steam.
3. They should have sent you an email with your serial number (they did it when I bought games on Steam, I've been using it for years with no problems). If you use that serial number with installer you can download on taleworlds site (trial version = full version), you can play without steam.
4. You can not reset serial number, once it has been activated on one steam account, its glued to that account forever.
5. Your M&B:WB serial number could be in the registry somewhere.
6. Steam should be able to help you recover your account if you get in contact with them and have any kind of proof of purchase for a game registered on that account, for example a copy of bank transfer paper or something like that. Might require more than that, I never lost my username, didn't recover it.
7. If you lose your accounts, write them down on a piece of paper and put it where similar papers are: passwords, site logins, emails, bank accounts, credit card PIN numbers... make a copy of that all and send it my way as a thank you gift
Hope you get your problem sorted out
Note: Steam might require a working week to reply, its a normal thing. Also consider its Saturday already and they might not reply till next Friday.