Diesel is usually shipped as a combustable which does not require buffers. I would have to see the shipping papers or placards to know for sure but my guess is that crude oil is shipped as a combustable also. No buffers are required for them then.

As for required buffers, if a train doesn't have enough cars to fulfill the buffer requirement more buffers cars do not need to be added. A unit Haz-Mat train by definition wouldn't have buffer cars in the train. (It is a unit train of only Haz-Mat cars) The buffers are added but technically not required. A locomotive running or not cannot be used as a buffer.

Some Haz-Mat only requires one buffer car while other require five. Some Haz-Mat shipments can't be next to other types of Haz-Mat shipments. A certain number of loads of certain types of one Haz-Mat shipments calls for speed restrictions. It gets complicated. This is not a can of worms to be opened here.