Hi Nick,
Basically the answers to your questions are:
The hours accumulated flying privately do count against your 900hr limit and generally you require the permission of your employer to allow you to use up some of your hrs!
Regarding flying a single engine plane on your days off, the licence allows you to do so, as long as your class rating is valid or in date, as per a PPL. So if a ATPLer has let their single engine class rating to expire, then they will have to go back to school and be re-tested to get it re-issued.
Hope that helps
Rb311