Well it wont be at the top of the 40 foot tower. The tower is less than a foot away from the eavestrough (which makes it easy to get on the roof) and they can stand on the roof while mounting the dish on the tower. I already made a mounting bracket for the installers out of 1/4" plate steel and will be bolting that to the tower before they arrive. It ain't gonna move. :-) The dish will then be at least 17 feet off the ground and can be easily accessed. (by me, not Mom, as she is over 80). I really want them to install it there as there are no other options and using a 6x6 post at the back corner of the property will not get the line of sight over the tops of the trees......
Your setup is similar to the one I did nearly 20 years ago when we moved into the current place. I had a ham radio 64ft tower put in (because I have lots of stuff to go on it). The SD dish is bolted to a steel plate, that's bolted to the tower a little under 20ft up the tower.
This ain't no crappy TV tower, so there's really no discernible wind sway even at the top, and especially not where the dish is. I'd say in the 20'ish years it's been up, the number of times we've had to worry about snow is maybe once per year on average, and it's usually on those milder wet snow days, and it doesn't stay long. I've never felt the need to worry about removing it. However, lower latitudes (where there might be a slightly more upward angle) could be more of a concern.
I think you have a fine plan, although the installers will probably be using battery-powered drills and aging drill bits. So you having your own set handy will be appreciated. I'd also recommend buying a bag of good quality, long tie wraps, and ask the installer to tie the cables to the tower legs every couple of feet. More than about 4 feet and, over time, the cables will loosen a smidge and bash against the tower on windy days. That may drive you nuts (and isn't really good for the cable). :-)
If you get to talk to the installers in advance, make sure you let them know what the plan is. Maybe even attempt to send pictures. Some installers may not have proper safety gear for tower-climbing and may not be comfortable with the plan. They do have the right to refuse to proceed if they don't feel safe, which will waste time trying to find a better-equipped installer.