For this question of yours, you are correct in most of the points you mentioned. You cannot control what will be the performance of the network and when there will be an outage. Now, going to your point about buffering and queuing of the Modbus data on Modbus master functionality. By default, Modbus Master functionality stores all the Modbus data on a database file which is stored in the RAM memory. It can hold up to 20,000 records and it will just overwrite the data once it is full. This point will give clarity for your doubt about the number of reads and writes the flash memory will store because first of all its not stored there :)
I hope this helps.