I wouldn't do that. True oil is linseed oil with a polymer added for hardness and drying. Tung oil take a long time to dry thoroughly where true oil will dry in a couple of hours allowing you to do several coats a day with steel wooling in between coats. Since you already have tung oil on the stock I would continue to hand rub with tung oil til you get the finish you want or sand it off and start again hand rubbing with true oil.
That I don't know. The reason I like true oil is because it has a polymer in the linseed oil and it is easy to touch up if needed. Perhaps you could call the company and ask the true oil people if it will work over another kind of oil.
Use Birchwood Casy Stock Wax it's a good water repellent and protector. I'm a sponsor here on the site and do this kind of stuff daily go to the sponser link and click on Warfields Dura Coating. Hope this helps.