Yes, you can buy tickets now, from Disney or from your favorite authorized ticket reseller, reserve your park reservations, and then apply the cost of the tickets you buy to the cost of the AP - it doesn't matter where you bought the tickets. However, you can only apply one ticket to one AP, so you can't buy tickets for two 4 day trips (two tickets per person) and apply the cost of two tickets to the cost of one AP for each person. And if one 4 day ticket costs more than one AP (LOL), you won't get a refund of the excess amount. However, you could save the second 4 day ticket and apply that to the renewal price next year.
I have also read more than once that people either received a refund or were able to apply the cost of the Memory Maker to the purchase of an AP, but I can't remember whether any of them were upgrading tickets or simply purchasing APs outright.
I can't answer your first question about whether you should buy tickets now or not, sorry!