I try to answer, based on my experience,

a) if you are not sure that the data you have downloaded by yourself from the broker are correct and gap free, the process you have described make sense.

b) I am not sure if training will generate different parameters based on the assetfix you are using. For sure different assetfix will drive to different test results.
I experienced different test results not only with different assetfix but even with different margin setting and the same assetfix.
In old z12 version <1.4 this behavior was very relevant. Not as important with the new version. I do not know why... before trading I test the z12 with 50, 100, 1000 margin setting and I compare the results. With my broker, higher margin generates better test results.

c)I re-train the z12 during normal trading hours just after having updated the assetfix with the download script. Spread and margin requirements are different in low liquidity markets on my broker, but this is broker dependent, I guess.

HtH