MWBC are commonly used to save cost. The load on the nuetral is the difference in the two loads. i.e. If the load on one circuit is 10 amps and the load on the other circuit is 5 amps, the load on the nuetral is the difference, 5 amps.
I agree. Put in a breaker box. The cost is relatively minimal and it will allow you to do more of what you want. Chances are that when you sell the house, if the buyer uses FHA or VA loan, they will require you to update the box to meet current codes anyway.
There are certainly a lot of situations where heavy loads are supported by wood and/or metal/wood supports, but putting wood between a jack stand and a vechicle that you are going to crawl under is NOT a good idea. You should always make sure that the jack stand is more than capable of handling...