#1. I’ve never had any type of trouble transitioning to my secondary weapon or transitioning to my offhand shoulder using an Aimpoint or Eotech. Nor have any of the other guys on our team that I know of. I’m not quite sure what you mean by “bulk”, both only weigh a few ounces. Most of our team has the Aimpoint lens covers opening down on the flattop, but a couple still have them opening up. When I first started using these sights I had the 1ST generation Bushnell Holosight and then a Comp ML mounted on top of the carry handle, still never had a problem. Best advice to avoid the possibility of this problem for the Aimpoint is to have the covers open down or remove them all together.
#2. As far as I know the Eotech is parallax free. I stopped using it a long time ago in favor of the Aimpoint.
#3. As far as fogging up, if you have access to either, place one or both in a freezer for a little while, take it/them out and exhale on the lens. You will still be able to see the dot/circle, dot through the sight. Are you familiar with Trijicon’s Bindon Aiming Concept? If not and you do have access to either sight, close the front cover on an Aimpoint or block the front of an Eotech. Shoulder your weapon (safely) with the sight on and mounted. Look into the sight with your weak/outside eye closed. Then close your dominate/inside eye, open your other eye and look down the side of your weapon. Now open both eyes and your brain will combine both views. This is the basic concept. You can still use both sights if the front was totally obscured by water, fog, mud, lens cap…etc. If you are not comfortable with that then all you can do is either get a quick detachable mounting system and remove the sight from the weapon or mount it on top of the carry handle and use the irons. (On top of carry handle not recommended.)
#4. The Eotech is a good sight. I just like the LONGER battery life, easier activation/ brightness adjustment controls (gloved especially), available mounting systems, and looks of the Aimpoint.
I hope this helps. If not maybe someone else can help me. Lord knows I need it! [:)]
Ray