Hi Vince, enamels do dry fairly quickly but they are not completely insoluble when first dried so you have plenty of time to feather edges and blend with a spirit-dampened brush.
Don't add linseed oil or Liquin to your enamels unless you're looking to make them dry glossy (it will also make the paint more transparent). Oil will slow the drying but I don't think the extra working time would really be worth having to deal with the shine afterwards, but if you don't mind this by all means experiment. A reliable matt finish like Dullcote would be one of the best ways to get an overall matt effect, airbrushed on near the end of the painting process; you'll then want to brush on something to give skin, leather, painted surfaces etc. varied sheens to make the figure more lifelike.
The best thinner for enamels and oils to maintain a matt finish is your basic hardware-store mineral spirits (white spirits if you're in the UK). You can get 'odourless' alternatives like Gamsol from Gamblin, or Winsor & Newton's Sansodor, which work similarly.
Hopefully someone with a little more recent enamel experience will also pop in here to help.
Einion