I agree with most everything above. I differ from daalter a bit in that I think within your PGs (except McLain, depending on his ATH, PE and BH potential) there are some viable future SGs. For instance, Sam has solid SPD, PE, BH and PA, with passable ATH. The only real weakness is DEF, and playing the zone with a good defender at the point will mitigate this. Land is a little less capable on offense, but is still passable, and his 90 DEF will really help your zone, and allow you to play poor defenders at the point.
In short, I think your guards are not bad at all, and have different strengths, allowing you to adjust based on opponent. For instance, against a press, you might go with Cortes at PG and Huff at SG (with 87 and 73 BH, respectively, along with great D3 SPD). Against a zone, you might sub Sam for better shooting, especially if the opponent doesn't have great scoring guards (to take advantage of the poor Sam DEF). Against great scoring guards, you might go with Land at SG and Cortes at PG, putting 45 and 90 DEF ratings on the perimeter, with Huff backup at both spots.
Now your rebounding woes will continue, but you can help them by playing - defense when advisable. You might also play Dunbar some at SF (even though Lanier should always start and play to "getting tired" fatigue - he's too good to ever sit much). Dunbar will be 2 spots from his natural C, but his REB could help in the zone, and his SPD, BH, PE are not too much of a liability.
At first glance, I think my base lineup might be:
PG - Cortes, Huff, Sam
SG - Land, Huff, Sam
SF - Lanier, Dunbar, Keller
PF - Cooper, Gonsales, Dunbar
C - Marley, Dunbar, Gonsales, Parker
In general, it looks like you've built a team of speed on the perimeter, and athleticism with some speed inside. The issue is that your zone defense is not best suited to take full advantage of those attributes: man, and better yet, press is. Not that having these skills in the zone is not helpful, especially the speed, but when recruiting (if you stick with the zone), you need to err to the rebounding side at the expense of a little ATH and SPD, and really keeping your eyes out for the very low DEF recruit with great scoring attributes. In D3, playing a zone, if you're not getting 1 of these every 2 years or so, you're wasting a significant advantage to the zone, and it looks like you've done this with Keller and Sam.
Good luck.