You should expose at box speed if you expect to retain shadow detail.
No developer will produce detail in an area that has not been exposed.
Diafine is a compensating developer, so it will mitigate problems with highlights.
BTW, in case you didn't notice, the instructions say DO NOT pre-soak. This is important.
Time and temperature are not critical. I ue five minutes in each solution, although you can get by with less.
Try to keep the temp around 70°-75°F, and keep all other solutions and wash at that temp.
As mentioned above, the working solutions last for years. Keep them in brown bottles, preferably in dark storage.
Replenishment is done by replacing the lost volume of Solution A with fresh, and adding the same amount of fresh
Solution B to its bottle.
Be VERY careful to avoid contamination. If you get any amount of Solution B in the Solution A stock, you have to
throw out Solution A, and since they come in a pack of both, you might as well toss B also.
I have separate mixing and measuring vessels, stirring rods, and of course trays for each solution. These are
separate from the items I use with other developers.
If you use a tank for processing, thorough rinsing after processing is sufficient to avoid contamination.