AVERAGEIF - Account for DIV/0, #NA and dynamic range

I would really appreciate some help with this. The attached spreadsheet has a thorough explanation of the problems I am having with creating these formulas. It calculates improvement in the weekly results of occupational therapy.
I have to create summary averages that can handle DIV/0 and #NA. The range of the formulas is dynamic ---- there is always a Week 1 but may or may not be a Week 3 or 4 result. Need to calculate % improvement from the 1st week to the last week the patient was seen (could be 2 weeks only).
I have a thousand words in this picture (at least) ---- attached is self explanatory.

I thought I had this solved once before but alas I did not. I tried to account for calculations that started in week 1 with a zero by using .0001 instead that drove the % Improvement way up and distorts the summary improvement.
Please see attached.
Thanks for the help.
Dave@ExpSC

You are correct. That is how it is supposed to work ---- improvement(or back slide)based on the initial value as compared to the final value regardless of how the patient does in between. I am challenged the fact that the final value is dynamic. Appreciate you taking the time to look at it and helping if you can.
Dave@ExpSC

This will give you the results you need for the Flex data and accounts for loss then improvement, or improvement then loss, constant improvement, or constant loss.

Formula:

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It will also mean the average in I15 doesn't have errors to contend with.

With the Ext data how do you want to reflect somebody who starts at 0 and ends at 3? Compared to somebody who starts at 0 and ends at 2?

More questions:
If somebody starts off at 12 Ext and gets to 0 (row 23) that is a 100% improvement. If somebody starts at 1 and ends at 0 that is also a 100% improvement. Are they equivalent?

Is the %age improvement per patient that critical, or is it just a vehicle to get the average improvement?

It's a good and valid question. I would go with primarily a vehicle to track Summary % Improvement.
Thanks
Dave@ExpSC

Does this work for you then?
OT Improvement.xlsx

I have changed the percentage of Ext to the total movement in column N and the percentage in N15 is the total movement/total week 1.

Wow - you are certainly good at this. These are a fantastic solution. I sincerely and greatly appreciate your solution to my post.
Many many thanks,
Dave@ExpSC