Use year has nothing to do with when you can go (any time of year) or when you can call to reserve (11 months out at home resort, 7 at others).
It does set the date by which you must bank your points into the next year if you want to bank. Currently, that date is the end of the eighth month of your use year, e.g., if you have a June use year, you must bank by January 31 in any given use year. You cannot bank any points during the last four months. The use year thus becomes important only in the situation where you set a vacation and then have to cancel. Assume that June use year (which runs June 1 to May 31) and you schedule a trip for April during the use year. Now assume you have to cancel come February. Points go back into your account but you cannot bank any of them because you are beyond the last banking date and thus points must be used for a trip that ends by May 31 of the use year or they will be lost. Assume that June use year and you reserve a trip for Ocotober. In August you have to cancel. Now the points go back into your account and you have plenty of time to use them or bank them
Thus, if you actually have a usual time of year that you will go, many recommend that you get a use year that will allow you to bank points if you have to cancel. For example, you say you would usually go in June or August. Ideal use years to preserve the banking/cancellation hedge for those months are June, April, March, or February (there is no May or Jan use year). Note, however, that even if you buy with that hedge in mind, it may eventually mean little because years from now you may decide that you really want to go in January every year because you don't have to work around kid school schedules any more.
As to purchasing jointly with parents, that is really a matter of personal choice. The advantages include that all would be entitled to any
DVC perks such as discounted AP's (perks are generally limited to owners and their immediate family living with them). The reasons not to have combined ownership involve mostly bad things that might happen in the future -- e.g., if one of the couples divorces there could be a forced sale of the entire contract; if the parent's die you may learn that they left their interest to all their children and you may have brothers or sisters who could then force a sale of the entire interest; if any of the owners has creditor problems, those creditors could attach a lien on the interest and force a sale of the whole thing.
