Error 1: Change the Estimated Finish Date to be later than the Actual Start Date, if that's possible for you.
Error 2: Yes, you can ask users to enter the estimated finish date or change the project type settings to not track the estimated finish date at all. So users won't have to enter estimated finish date.
Error 3: You can select the 'Track estimated time..' setting in the Cost and Effort tab in the project type. Once you select this option, the percent complete will be automatically calculated by PPM and users won't need to enter it everytime or at least for the first time when entering actual for a task.
Error 4: This issue is fixed in SP6. If you upgrade to SP6, then this should get resolved by itself.
We have been struggling with Error4 above since SP4 was installed. We thought the work around was to enter data for only one task at a time and then SAVE before entering hours for the next task on the time sheet. That seemed to work, but was aggrevating for the folks entering 500+ time sheets a week. While it was reported as solved in SP6, it was not. We have been working with support for 2 months now istalling patches to logfile functions for them to gather the appropriate information. We have just recieved today a new patch to the logging process and they indicate that no one else has logged any support issues related to this error. We have had to escalate this several times to get any traction at all. If you are having this problem, please enter a support ticket. If you have, then this is just an additional demonstration of a lack of co-ordinated effort or honesty from the support group. These time sheet entry problems are killing our adoption of this tool set.