While you can coat wood with a suitable finish to mitigate the effects of internal moisture changes as humidity goes up and down, for all intents and purposes consider it impossible to prevent it entirely.
The finishes that do the best job at reducing the effects are film finishes, which as their name suggests literally create a film that covers the wood. On a practical level what you get with a reasonable coating of a film finish is a reduction in the maximum and minimum moisture levels of the wood, reducing but not eliminating seasonal movement.
So in general you do always have to design with seasonal movement in mind.
I was discussing this with a coworker and he suggested that the wood needed to be treated with tung oil
Tung oil, like all true oil finishes, is actually a very poor 'sealer' for wood. It provides little or no barrier to water vapour in addition to its well-known poor protection from liquid water. Tung oil is a bit of a thorny issue in the finishing world because of misuse of the name, so it's possible your co-worker may have been referring to a product like "tung oil finish" which is something very different indeed.
The original and most famous Tung Oil Finish contains no tung oil at all! What it is instead is a type of blended finish similar to "Danish oil" and "teak oil". These are all oil-and-varnish blends, diluted with additional mineral spirits to make them thinner and easier to apply. Note: you can easily make your own version of something like this, mixing a regular varnish and your oil of choice approximately 50:50, then diluting this as much or as little as you like (most commercial versions are much thinner than they need to be).
However, you can do something else to help minimise the effects and that is to select the boards carefully from the outset, taking advantage of the fact that wood does not change dimension equally in all directions:
What this means on a practical level is this: if you're using solid wood, choose boards that are quarter-sawn or rift-sawn and they will expand and contract across their width much less than flat-sawn wood of the same species.
Note that the difference is said to be about half but this is just a very rough guidelines, it's more with some species and less with others. A full table showing the relative numbers is given in this recent Answer.
Not using solid wood
Another possible option to sidestep the issue is not to use solid wood. Plywood, particleboard or MDF can be used instead in specific circumstances, and these essentially don't experience any seasonal movement.
So, do I need to pre-seal the maple wood somehow before I even assemble it? If so, then what is the best method to do this?
Of the practical finishes you can use to 'seal' the wood the best are shellac, varnish or lacquer. Since lacquers almost always require spray equipment to apply which I presume you don't have you can cut that list down to the first two.
You can also do this very effectively with an epoxy coating, but that's not commonly used on indoor projects and is usually very expensive to boot.
If you're using shellac or varnish you want to apply at least three coats to take advantage of their protection, but 5-7 would not be too many.
Also, on the outer part of the inner leg (see project link below) it was suggested by the same person that I put a wax or waxy layer on the shaft to ensure smooth leg operation. Would you agree?
It certainly can't hurt. Wax is widely used in woodworking circles for its excellent slip qualities, used to help reduce friction on saw plates, plane soles, the beds of jointers and table saws, and on drawer runners.
And if so then what would you recommend for this?
A coat of paste wax, buffed to a shine.
Note that you can also make your own version of this if you'd like to, guide here.