Formula field help

I have a task form to assign specific tasks to individuals to work on them. I have "Estimated Hours", "Actual Hours" and "Remaining Hours" which is a formula (Estimated_Hours-Actual_Hours). I also have a Project Length lookup field that's a in which you choose how many weeks/months you have to complete the project and it will then formulate the Weekly Hours =(Estimated_Hours / Project_Length).round(2) which will break down how many hours a week you need to devote to the project to finish it. My questions are:

1. If nothing is selected under "Project Length" how can I default the "Weekly Hours" formula to calculate (Est. hours/1)?

2. If "Weekly Hours" is more then "Remaining Hours" can I make "Weekly Hours" = "Remaining Hours." Otherwise if only five hours are left until the estimated completion of the project, have Weekly Hours read as 5 instead of 7 or whatever the Weekly Hours is calculated to be each week?

3. Can I somehow round "Weekly Hours" to .25, .5, .75 or .00 instead of (example: 5.68 would read as 5.75)?