I'm guessing as long as it looked like Steamboat Willie and not the modern Mickey you'd be fine as well.
Two separate things, really.
From a copyright perspective, all that is in the public domain are the first three Mickey Mouse films. Attributes of the character in those films -- appearance, behavior, etc. -- may be used. Copying other attributes from still-copyrighted sources is not permitted, such as the particular voice, the color appearance of the character, etc. As those sources drop into the public domain too, over the years, they'll also become available.
But from a trademark perspective, the issue is whether customers may be confused as to the origin of goods bearing a Mickey Mouse logo (either as a character or even the word mark) -- are they from Disney or somewhere else? Also, whether junior users of the mark are diluting the uniqueness of the unquestionably famous Mickey Mouse mark even where there is no confusion by customers. This is typically framed as either blurring -- associating the mark with goods it's not normally applied to -- and tarnishment -- weakening the mark by associating it with unsavory, unflattering, or inferior goods.
So when you say:
Could Disney trademark the Steamboat Willie image since they have used it, and continue to, to this day on merchandise, etc.?
They've already done this. (Also, even if they hadn't, Disney isn't losing rights to use Mickey Mouse themselves -- they're just losing rights to control others' use of it)
So you might be fine if you made say a movie but not plush toy?
This brings us to the last couple important concepts: 1) Trademark is not a substitute for copyright; 2) When a copyright or patent enters the public domain, it can have the effect of causing a trademark on the thing that is now in the public domain to be lost through becoming generic.
Let's tackle the second one first: The point of a trademark is to give consumers assurances that goods marked with the mark share a common origin and are of the same level of quality (whether good or bad is irrelevant; the point is consistency). So if you see a bottle marked COCA-COLA, you know it must originate from the Coca-Cola Company and that it's going to taste like any other Coke. If Pepsi starts to sell soft drinks labeled as COCA-COLA but containing Pepsi, this is infringing because it confuses customers: they don't know where a given marked bottle comes from, or what's in it. The Coca-Cola Company can sue to stop this. But if they failed to protect their rights, they'd lose them because they would be allowing confusion to run rampant, and confusion is incompatible with a viable trademark.
In the late 19th century, Henry Perky invented shredded wheat and got a patent on the food itself, and on the method of making it. Since no one else could make it or sell it due to the patent, SHREDDED WHEAT functioned as a trademark; anything marked that way must come from Perky (or Nabisco, who ultimately acquired the rights), and would be consistent in quality. When the patent expired, rival company Kellog's started making it too, and also used the SHREDDED WHEAT mark. The US Supreme Court held that this was not infringing because with the expiration of the patent, everyone was free to make it, and call it by its name. Thus, the loss of the patent killed the trademark.
In a much later trademark case, the Court also held that trademark could not substitute for copyright; once a work is in the public domain, trademark cannot be used to prevent the public from using it.
Together this means that anyone can start making new Mickey Mouse films derivative of the public domain ones, starting next year, and using the Mickey Mouse character by name. This will kill the trademark, at least in the realm of creative works -- books, movies, comics, etc. Maybe sculptural works too, like plush toys (since the Mickey Mouse shape is the point, rather than a source identifier -- think of the difference between a shirt with a picture of a polo player and a shirt with the POLO mark on it). But not non-creative works, like Mickey Mouse-branded ice cream bars or wrist watches.
Certainly there's nothing that prevents the use of public domain characters as trademarks, like PETER PAN branded peanut butter, or KING ARTHUR branded flour. But equally, those trademarks don't prevent anyone from making new books or movies or what have you about the characters of Peter Pan or King Arthur.
All that being said, Disney has loads of money and is famously litigious, so it'll be interesting to see who goes first to test the waters.