Really, the answer to this is entirely dependent on the individual. However, I'm getting that you have the ability already to play guitar fairly well by ear. You can probably hear a melody and put a few chords together for it, and you know most of your basic guitar chords. That's a good place to start.
If I were your teacher, I would strongly suggest that you learn to read music halfway decently if you can't already (because yes, it will help you learn faster and easier). Once you get a working knowledge of notes, you can begin leveraging your guitar skills by working on your own arrangements, picking up fake books, and the like. Reading notes is a skill that will expose you to a whole lot of new musical ideas, just as literacy exposes you to a whole lot of general ideas. Sure, it's easier to play by ear than to read music, but it's easier to learn to understand speech than to learn how to read as well. Musical literacy has analogous benefits to "book learning."
If you'd like to look into this further, I like Alfred's Adult All-In-One Course. (Here's a link to a pdf someone uploaded, although I would suggest that you buy the book and do all the theory exercises.) You can blow through the first 30 pages quickly, probably in a couple or three days. Make your goal to be to play all those silly songs perfectly (don't obsess over the stuff about how to sit and how to hold your hands; those are good basic ideas but there are all sorts of other opinions on this stuff). As you improve your play, you'll find more interesting material to work on. (If you blow through the entire book, you'll probably find that your $10 or so investment gets you to at least the place that that $75 investment for "Instant Piano etc." would get you. And there are several more books, if you like the approach enough.)
You'll notice that all the songs are spelled out. You need to be able to play as written, but you will find it interesting to work out your own arrangements of the music. By the time you get to book 2 (or perhaps before), the chords are written over the music, and you should see a song or two that you can play that is sufficiently interesting to get you thinking about how you can change the voicings of the chords and experimenting with altering the harmony. Once you get to that point, I think you'll be where you want to be.
What's nice about all that is that you'll find that your guitar playing improves along with it. The piano is like a map; every note looks the same for each octave. For example, on the guitar, if you want to play the lowest B it's the first fret on the A string. An octave higher, and it's the fourth fret on the G string. On the piano, every B is the note immediately to the right of three black keys (and conversely, every note to the right of three black keys is a B). So, it's much easier to figure out how to play a chord on the piano, without having to resort to learning it by rote as you generally do on the guitar. As you do that, you'll get a clearer picture of why you play the notes you do on a guitar chord, and this will allow you to invent alterations in your chord voicings on the guitar as well.
