The drawback in adjusting your supplies is that your enterprise statements will not reflect the cost of all of the supplies. I normally recomend allocating remaining supplies between all of the fields that they would have been used on or if you are using an "Overhead Icon" allocating them to that. This will result in the cost being passed on to the fields. Adjusting can be used, however I would only use it when the amounts are small.
Splitting a field will only split the enterprise that is open in the field. Any enterprises that were previously closed will not be split.
The Beg. vs. End accounts are used in valuing inventories on your accrual books. They have no effect on your cash books. As you add costs to fields the costs will be put into the inventory accounts with the offset (a credit) going to the Beg. vs End account. This credit is used to reduce all of the input costs (chemicals, seed, etc) that show up as expenses. When the crops are harvested and sold the inventories will be reduced with the offset again going to the Beg. vs End (as a debit). The Beg. vs End account allows your accrual books to give you a true profit for your farm where your costs are not taken out of your income until you harvest and sell your crops. These accounts will be automatically adjusted by Farm Fund$ and you should not need to ever use the accounts in any type of entry.