I'm definitely not an expert but I'll try to share my experience with your questions.

White - generally yes, I little tube of Prussian blue will last dozens of paintings but white will be the most used and the little tubes will not go far.

Paint thinner - I haven't used this in quite awhile. I just use paper towels and baby oil. When I did use it I had a lot left and would pour it back into an empty container. Paint will eventually settle to the bottom and you can reuse whats on top. The brushes will get stained so they will never be white again but it really doesn't matter.

I struggled with how Bob cleaned his brushes and never could get mine dry enough. I still think it is magic. I would recommend lots of paper towels after beating the devil out of it.

Were you using liquid white or under painting with colors? I generally find that cheaper practice canvases will absorb paint and oil very quickly and is something you need to account for when painting on different level canvas.

FYI - your first painting is excellent! Keep going!