Here's my take on your second question. Long story short, it is naturally simpler to "divide & conquer" an odd group:
In an odd grouping, the center item likely becomes the natural focal point, and so you'll get a mental image of that item in your mind as you look at it. Next, your eye will likely gravitate to one of the side groups to compare your mental image against items from that grouping. If you like an item from that grouping better, then your mental image is replaced with that item. If you don't, your mental image will be refreshed when you glance back to compare it against the items from the other group. When you get to the other side, you've already developed a mental strategy for comparing groups of that size, so you reuse or improve upon your strategy for comparing items in that group, which lessens your mental burden. All in all, the biggest sub-group size you had to consider during this process was
(N-1)/2, or for example 1, 2, and 3 items per group for 3, 5, and 7 items total respectively.
In an even grouping, there is no clear center (except whitespace), so the left or right item will probably become your focal point, likely depending on your reading language. Again, you will get a mental picture of that image in your mind and then compare it to the remaining items. At this point, the two obvious choices are a linear or a grouped comparison. If you linearly compare items one-by-one, there will likely be a lot of back and forth eye-movement to refresh the mental picture of the item you like and then return to the item you are considering, and so your eyes and brain will get an extra workout. If you comparing items as a group, though the remaining group is odd, it will be difficult to compare it in the same way as you did before. This is because you already have one extra thing you are thinking about (the first item), and you will also not have the visual symmetry in your groups, because the first item will throw off that balance and be a distraction. Either way, you will lose the ability to re-use/improve upon your mental strategy for comparing groups. And last but not least, your maximum sub-group size will be
(N-1) instead of
(N-1)/2, so you will be dealing with larger than necessary sub-groups.