Hi Cliff
I am sure you will get a lot of different opinions on this one.
Here is mine
You don't have to put clay in your pond it is just a good idea.
I add clay to my pond and the sand filter catches it rather quick, so just get your timing right like on Sunday afternoon, add the clay, remember smaller amounts more regular is better, when that clay is in your Sand filter it still works for you, so add it just after you have cleaned your filter.
To get it out, well that is no problem now days with that conversion kit from NGB.
As for the alkalinity, its major purpose is to counter a PH crash, but nice to have it sitting at 60 t0 100. Here the use of oyster shells works best, but I have also found that Gypsum cast into blocks and placed in a stream will do the trick, I t is easy to make PM me I will tell you. I also add some Limestone to my gypsum mix