Tom, randomly choosing Sept 19, I found $513 USD for the all-inclusive roundtrip. Not inexpensive by any means, but potentially hassle free and might offer some hotel savings (outside of the Belmond package) by not having to pay the higher cost to stay closer to the site.
But, I can understand if you like the closer-to-the-local ways of traveling. There are advantages to both approaches. Only YOU know what is right for you, and this is sure not your first "travel rodeo." You have the experience to know what is right for you, and you have done your research.. Our travel approach has evolved a lot over the years, as we have aged, to make things a bit more seamless and comfortable..
I made the suggestion, thinking POSSIBLY the train might offer the all-inclusive experience you are looking for once you arrive at Machu Picchu (we had choices on the extent and physical difficulty of tours..we chose the more difficult, extensive route.).
Your posting and the suggestions you have received will help others evaluate their plans.
And, of course, outside of the changes with pre-booked tickets, there is always the risk of a heavy rainfall or other natural event that could cause disappointment with pre-booked tickets. The day we were there, there was a light sprinkle of rain and significant fogging. I was sooooo disappointed "we've come this far and blah blah blah..........." But, when we got up to the part where one overlooks the Inca village (or whatever they call it), the rain had stopped, the clouds parted, the fog lifted, and it turned into the most beautiful sunny day.
Do beware that if there is any moisture, the wooden platforms/steps that help to connect the path of the old stone ones can get really, really slippery, even when one is wearing study hiking shoes with excellent traction. My husband slipped down about 8 stairs (he was ok), and then a young guide (with a group other than ours). slipped into me with the back of my ankle keeping her from sliding down further than the 4-5 stairs she had already gone down. I have what some could consider really long legs, but even for me going down some of the old stone steps was more of a "controlled drop" vs. a step down.
It's an interesting place, and you will be happy to have experienced it.
Safe travels, and let us know how it all works out for you.