How can I make sure a student doesn't see the grade for an assignment until I am done grading?
You can set a grade posting policy for either all assignments in the course(including quizzes and any other graded item) or on a per-assignment basis. The settings for either process are located in the gradebook, see linked articles for screenshots. Be aware that this method requires the instructor to manually select "Post Grades" when you are done grading. Canvas will not infer when you are done.
This feature does not work exactly as expected with New Quizzes. In the case of a New Quiz that has some or all auto-graded questions (usually multiple choice), the student will be able to see their "initial grade" - based on the number of auto-graded questions they got right as a fraction of the total points on their Quiz Results Page (see image) even if the instructor has set the grade posting policy to Manual. To avoid this, you must adjust the Quiz Settings for the individual quiz. Specifically, you should Toggle ON the "Restrict student result view" option
When an assignments allows multiple submissions, can each attempt be given a grade?
The instructor can grade each attempt, and toggle through all of the student's past attempts to see past grades and comments. The instructor can even grade a past attempt after a subsequent attempt. However, the grade that will "stand" in the gradebook, and the ONLY grade that will be visible to the student is the most recent one. Furthermore, the student cannot view any information at all about previous attempts. So, if you'd like a student to be to have access to initial drafts of an assignment and your feedback on those drafts, then each draft should be set up as a separate assignment.
How are the weights calculated within an assignment group?
You can weight final grades based on assignment groups. Selecting this option assigns a weight to each assignment group, not the assignments themselves. Within each assignment group, a percentage is calculated by dividing the total points a student has earned by the total points possible for all assignments in that group. (In other words, assignments worth 100 points are worth more than assignments worth 50 points within that assignment group's calculation.). It does not appear to be possible to "normalize" scores within a category - in other words, to make all assignments within a group have the same weight regardless of number of points. This is has been submitted as a Product Idea.
Is it possible to create the Gradebook from from an Excel spreadsheet (i.e., can we add an assignment to Canvas via an uploaded CSV file)?
Yes!
Read this Canvas Guide for details. (There is fine print to be aware of. For example, assignments come into Canvas already published and you will need to update settings such as due dates). The linked guide includes the specific column headers you need to use and other guidance for creating the CSV file in Excel.