Answers to your questions are:-
1. No. If you came on a visa and never made an application to the Home Office, they won't have a file on you. No point in creating one.
2. It's quite possible that you will be served with a notice as an overstayer. Probably your fingerprints will be checked to see whether you've previously claimed asylum or come to notice before. Make sure you check in with plenty of time to allow for this, but they are unlikely to prevent you boarding your flight. Being listed as an overstayer doesn't in itself trigger refusal of a spouse visa application.
3. A little more difficult. If you produce evidence of illegal working, the ECO might attempt to refuse you under Para 320(11) of the Immigration Rules, i.e. that you contrived to "Frustrate the Intentions" of the Rules. Have a read of this:
http://www.ukvisas.gov.uk/en/ecg/refusa ... onsofrules
You will note that they can't just refuse you for the immigration offences of overstaying or illegal working, there have to be "aggravating circumstances", such as using a false identity. You'd better look at the evidence you propose to produce and take advice as to whether it is likely to assist your application.
Two things interest me, if you don't mind commenting:-
- why do you think explaining reasons for overstaying will make any difference to your spouse application? The important thing to show is that you've been married for 5 years and it's a genuine relationship.
- you got an ETD, presumably from the ZWE High Commission? What evidence did you have to show them to get it?