Oil and water don't mix unless it's done with specific ingredients under specific conditions. That is the ONLY reason why actual pre-shave oils cost up to 10x the price of just plain olive oil or castor oil or any other oil. Pre-shave that is designed to be used prior to shaving will be thin, easy to spread on a wet face and not clog your pores, sink, brush and razor. It will be long-lasting because you don't need tonnes of product to cover all the areas you need to and it will still be effective. Normal oil won't do any of these. It's why I disregard complaints about the cost of pre-shaves compared to normal creams/oils/soaps etc. So to answer your question...no. In my opinion I don't think diluting it would be effective at all. I tried with olive oil and it just doesn't work as well and creates more clean-up. Definitely not worth the hassle (especially since this could reduce the quality of shaves if it is thick and greasy and therefore sticky). These companies have been around for decades selling these types of products since they began. They'd know if there was a market for it to be used as something else. Again, just my two cents though...for a few extra dollars you can get american crew ultra glide shave oil...its fantastic as a pre-shave (is my go to) and can be used on its own when in a rush.
Hair oil is a bit different because your hair and scalp both absorb quite a large amount of oil, even more if you have thicker hair and even more if it is curly/wavy. My family has been using coconut or almond or argan oil in their hair for generations. It is like the most common hair therapy ever. If your hair/scalp is feeling dry or if there are issues with dandruff or you use a really drying hair product like clay/pomade then these are good to put on at night and shampoo out in the morning. Unless you use too much...it shouldn't make your hair look greasy but there will be a somewhat unnatural shine and 'hold' to your hair. Always use a towel wrapped over the pillow though. If you use it in the morning, you only use a few drops.
This is why most people cast aside pre-shave oils. In the beginning everyone used AoS which was basically just 'hip' at the time. That was not a pre-shave oil..it was well..oil. There are plenty of pre-shave oils which are specifically manufactured to not be greasy, thick and scummy. These oils are worth it (for my face at least) and function completely differently from plain oils. They are actually slick, lubricating, protective and moisturising WITHOUT feeling like you just wiped your face in oil slick.
I have thick, straight, and heavy hair. I've been using almond oil and American Crew hair fiber to style my hair and it works a treat. I gather a little bit of the hair fiber from the tub in one palm and then pour a little bit of almond oil into that palm before mashing and mixing the two into a paste with both hands. I then apply that paste in my hair using both hands.
