3. Just how difficult and dangerous would it be to mix the Platinum Stock solution... Is this done with ingredients at room temperature?

Mixing the platinum stock is pretty easy and is not hazaradous, but for very small amounts you will save virtually nothing by mixing it over buying the solution already mixed. What you want for the platinum stock is sold by B%S as Platinum Solution #3 (or for palladium toning there is Palladium Solution #3). Both are 20% solutions, i.e. 1g of metal salt in 5ml of water.

A standard toner for toning kallitypes and vandykes consists of 10g oif citric acid and 5ml of either the Platinum Solution #3 or Palladium Solutin #3 in 1000ml of water. This may also work for regular silver gelatin papers, or you can use the Marshall formula which is in fact very similar. It is best to use the solution one-shot, using the minimum amount needed to tone a print. For a kallitype this amounts to about 200ml for a 12X20 print. You really need a flat bottom tray to tone with this small amount of solution.