1.
When a work item (To-Do) is created, the standard unit will be the same as set for the project.
When creating a task without setting the value for estimated time, the user can change the unit (hours, days, weeks, %) when changing the task later.

Let the user set the unit of a task while creating it initially, not only when editing later

2.
When a task is set for a whole week but estimated time is set to 8h the users availability will show 20% for that task (40h/w)
the task will show 8h planned.
When a task is set for a whole week but estimated time is set to 20% the users availability will show 20% for that task (40h/w)
the task will also show 8h planned.
At first it seems that this does not make any difference.

However, when the user has a day off that week, things get complicated.
Now the results will be the following:
When a task is set for a whole week but estimated time is set to 8h the users availability will show 25% for that task (40h/w)
the task will show 8h planned.
When a task is set for a whole week but estimated time is set to 20% the users availability will show 20% for that task (40h/w)
the task will also show 8h planned.

So the 8h in the first scenario are different 8h in the second scenario.
Two suggestions for that:
Show the planned hours in the unit it was set up with (% or h) and put in a calculated value based on the planned events in the ressources calendar
8h in a week with 4 days off is 100% but 20% in a full week is also 8h. This should be somehow distinguishable