Fred is a working ME, and I am not, so generally if Fred has an idea, I second the motion.
I think the examples Fred shows in the photos would be the way to go, at least for an initial engine, since their concepts have been carefully thought out, and they were actual commercial grade, well functioning engines.
Fred's examples are also very similar to what it sounds like you have, and I am afraid that if you get bogged down in a double acting engine, you will not have a good working design.
I would say if you want a double-acting engine, then perhaps design one with that intent, and you will have a much better chance of succeeding than modifying what you have.
I saw a photo of an engine made from a refrigeration compressor, used on a launch.
I will find that and post a link. Very similar arrangement to what you have/want.
I would err on the side of a slower speed, as you know that will probably work.
If your speed gets too high, you may have all sorts of issues, such as your shaft coupling flying apart, alignment issues which cause a problem from too much vibration, etc.
The piston valve across the top of the engine is an excellent idea in my opinion, and is not that difficult to make. I actually used a similar valve on my first single-action steam engine that I made in high school.
The trick I think will be to get something working, and then operate it in somewhat of a "safety" mode, and then modify/adjust as necessary to refine the design to its final configuration. It will certainly take a little tinkering.
I am off to find the link I mentioned.
Here is a link to "The Steamboating Forum", which I find very helpful since these guys (like Fred) operate full sized working engines, not scale models, and so have real-world experience with them. Unfortunately, a lot of my knowledge is book knowledge, but I have build steam engines and boilers, and operated both.http://www.thesteamboatingforum.net/forum/viewtopic.php?f=8&t=523&start=20
And here is a link to the page of the guy who uses a refrigeration compressor in his boat:http://smaalders.net/barts/boat.html
And a photo of his engine:http://smaalders.net/barts/engine.html
I think a wise thing for you to do is email this guy and pick his brain, since he has a working design, and that could save you countless hours of design as well as maybe save you from making a design that does not work well.
As far as steam on aluminum, I see no problem with that.
The only problem I have had with steam engines is forgetting to flush the condensate out of the cylinder after running the engine. I put the engine in storage for several months, and when I tried to used it again, the piston was rused solid to the cylinder wall, requiring a total dismantle/cleaning.