Hi,
When I filled in Form 80 I had a lot of the same issues, not remembering dates etc. I filled in what I knew and put down approximate dates for the rest, I am in the Navy so had a lot of countries to list, I tended to put down the month eg Jun2006 - Jul2006 rather than an accurate date and there were no questions asked by my CO.
For the address parts I listed my future work address and I put down a guy who has been dealing with my transfer as a contact.
For Q39 I just put to take up employment in the.............on the date of.............etc, I was as honest as I could be about dates and left the rest blank or put unknown.
My CO never queried any of it so hopefully it would be the same for you, I think as long as you are honest there will be no problems, if you don't know the answer don't worry.