Do these filters have an expiration date?

No, they do not.

are these replacement filter for the seychelle 38 oz canteen?

Yes they are

What is the best way to clean the filter after each use?

You do not want to clean the filter you can clean the bottle with a mild detergent.

I have an old Seychelle water bottle, I believe it is 28 oz. Would this filter fit my old bottle?

Yes the filters will fit the older bottles as they style of the bottle hasn't changed

My bottle is many years old will a new filter still work in it?

Yes as long as it is a 28oz flip top or 24oz pull top bottle

I'm headed to Zambia. Is this filter good enough for their parasites, and other waterborne ailments? Never used one before. Don't want to be sick for 14 days!

Yes this filter will work. This is just the replacement so if you do not have a bottle you will need to purchase it. here is the link to the bottles Seychelle Advanced Water Filter Bottle

How many uses before replacing filter?

It has a filtering capacity of 100 gallons.

I understand this filter fits the 28oz bottle, but what is the net volume of water actually filtered per fill-up, deducting the displacement volume of the filter itself? Thank you!

I would guess the bottle would hold 25 to 26 ounces of water with the filter in it.