Easy question, first!

There is no
DVC discount on any major park admission media except the Ultimate Park Hopper (formerly known as the length of stay) pass. There is a 10% DVC discount on the UPH.
You are correct in believing that use year has nothing to do with when you can call to make a reservation. That is always 11 months ahead of your desired check out date for your home resort(s) and 7 months ahead of the desired check out date for non-home DVC resorts.
Use year is
only important because it determines banking deadlines. You can bank up to 100% of your annual allotment of points if you do it before the end of the 6th month following the start of your use year. For example, if you have 200 points and a February use year, you may bank up to 200 points if you do it on or before July 31. You may bank up to 50% of your annual allotment if you do it by the end of the ninth month(October 31 for a February use year) and 25% if it's done by the end of the 10th month( November 30). You may not bank at all during the last two months of your use year. The percentages are cumulative. Continuing with the 200 point example and a February use year, if you bank 125 points in July, you cannot bank any more points after July 31. That's because 125/200 is more than 50%.
Banking deadlines/Use year comes into play if you find yourself needing to cancel a trip. Let's say you scheduled a 200 point trip for March (and have a February use year), and had to cancel. If you do it more than 30 days ahead of arrival, the points are returned to the current use year. In that case, you have until January 31 of the following year to reschedule your or you can bank the points into the following use year (because you have until July 31 to bank your entire allotment). If that same trip was scheduled for December and you had to cancel, you would have to reschedule the trip (or find someone to rent the points for a stay) before January 31. If you couldn't do that, you would forfeit the points. Since it is so late in your use year, you could not bank any of the points.
Remember, this is only important if you have to cancel your trip. Hopefully, that will happen very rarely or not at all. Also keep in mind that your travel patterns may change over the course of your membership. What might be a "good" use year now, may not be so "good" then. There are other "rules/deadlines" about canceling in addition to the banking deadlines, but this post is getting too long as it is, LOL!
If you normally travel in February and July/August, a February use year would be good for you. However, if you purchase from Disney and want a February use year, you would not get any 2002 points. That makes the purchase less valuable than one with a use year that would give you 2002 points. You would have to decide if no 2002 points is worth getting a February use year.
That was long. Hope it made sense., If not, ask more questions.
Good luck with your decision!