For years, my jousting crew and I mounted our horses in armor, which sometimes weighed up to 70 pounds. (not me, the men--my armor never weighed that much) Our horses HAD to stand quietly while we mounted. It would be impossible to hop around swinging on a moving horse with all that weight.
We practiced many ways of training our horses to stand quietly while being mounted. What worked best for us (and seems doesn't work so well for you), was giving the horse a treat when it stood quietly to be mounted. For some of the horses, it was a gradual thing--the horse got a treat if it stood for only a few seconds, and we slowly increased the time. If a horse got mad because it thought it should have more treats, oh well, we were mounted and we got on with the business of riding. I think the idea of clicker training is a really good one. The horse will associate the clicker with the treat.
My next door neighbor is 76 years old and in the last 3 years has started having difficulty mounting. There is no shame in having someone hold your horse while you train him. It might take 6 months or a year, but everybody is safe. If you have people around, nobody minds holding a horse to help somebody out.
My neighbor has been riding his whole life, and he doesn't like depending on others to hold his horse. Here are some ideas that other elderly riders have done that have been successful. One man keeps the halter on under the bridle. He slips the leadrope through a ring on his trailer and holds the rope as he mounts with the horse "attached" to the trailer. This needs to be practiced some before attempting to mount because a squirrelly horse is not going to stand quietly, and elderly riders have to have a rock steady horse to mount. So, practice first with the horse on the rope "attached" to the trailer.
One person hangs a bucket with some feed on a post or on the trailer. While the horse happily eats, the rider swings up.
I know another elderly rider who slips the rope through a ring on a snubbing post. It takes a bit of practice to get this to happen smoothly, especially with a horse that already doesn't want to stand quietly to be mounted. I think clicker training would come in handy in this situation.
Another elderly man clips the leadrope to the trailer ring, mounts, and reaches forward to unclip after mounted.
I've never been a fan of backing up a horse that doesn't stand to be mounted. I've found it just causes the horse to back up when you try to mount.
I also am not a fan of making the horse "work", moving the feet etc. when they don't stand to be mounted. It has been my experience that in this case, "moving the feet" makes the horse anxious and angry, and less inclined to stand quietly.
I really like the idea of going in front of the horse, praising him, petting him, getting yourself in a positive, grateful, accepting frame of mind.
Having a horse holder is the best solution in my opinion. The horse holder can have some treats. I have found that horses learn fairly quickly to anticipate a treat when coming to the mounting block. Once again, if the horse is mad when the treats end, well, you're on the horse, ride off. Good luck. I've had a lot of experience with teaching horses to stand quietly to be mounted. With patience . . . treats . . . and time, they come around.