1- Any archer that uses the tower always gets headshot easily by a decent archer because you can see them at all times and they take time to walk up the stairs. Therefore.. not a viable strategy.
2- What the hell would anyone actually be able to do from the palisade that has very bad view towards the front where the opposing archers would be.
3- The spawn that has less cover has a higher wall at the center, so it can be used instead. A fair tradeoff.
4- Not sure what you mean by being able to shoot through the palisade wall, I've never seen that happen, the closest I've seen to that is people not putting their head behind the pike thingy's at the right angle, so they get their ear torn off by an arrow. (Which counts as a headshot)
5- I highly doubt I would be getting the first head shots of the games if I were to camp at the pallisades, there's simply no angles towards the opposing spawn. I spend most my time up at the wall because there is a much greater field of view and a bunch of dum dums who think they won't be unlucky enough to be the first ones targeted, so they don't even put their shield up. A bunch of easy kills, but that's all POM. I've never played on Arena competitively, so I can't REALLY say how that would work out, but I am quite certain that the same POM strats simply won't work.