To answer your questions;
Reverse engineering presets seems like it might be a good idea but I don't do much of it, just never really think to.
Learning every wave in massive seems like a waste of time, as Dubunked said, focus on the basic ones a bit and leave the rest.
I also think you would benefit from working with a synthesizer other than Massive to begin with, Massive is mostly pre-made wavetables with some interesting PM/RM options, not really my idea of a beginner synth but to each their own.
I would highly recommend Sytrus because of it's versatility with many types of synthesis, although it is a bit daunting to look at sometimes, probably not good for a beginner either.
Start out simple, maybe massive will work for you.
How I learned about sound design;
First I read about wavetable and subtractive synthesis then moved on to more complex concepts like the various angle modulation based synthesis types (FM, RM/AM, PM), from there I read about additive synthesis and just kept going about learning how each one works then applying that knowledge to try and remake sounds I liked/had ideas for.
From that point and sometimes during learning about synthesis I learned how various effects (EQs, compression and limiting , waveshapers and other distortion types, convolution, reverb and delay, filters, phasers/flangers, ect...) worked.
Youtube tutorials can be nice but a lot of them are just telling you how to make a specific sound without actually telling you why or how that sound works, I pissed around trying to learn legitimate knowledge about synthesis from Youtube before I started just googling things and taking a few hours to read about synthesis from one ancient PDF or another...
One thing to understand is that most (I can't speak for all) sounds are relatively simple to design, it just takes time figuring out how they work to do so.
Also, you may already know how something works but you wouldn't be able to figure it out by listening to it, this is where experience comes in to play. It took me some time to differentiate a triangle from a sine wave when it was placed in a mix with a bunch of other sounds, I know it sounds silly but if you haven't allowed your brain to identify certain sounds, all the book knowledge in the world wont get you anywhere, trial and error is essential (or as they say "practice makes perfect".)
Another thing that frustrated me as a beginner, was notes played in chords, I would try to remake a sound only to find out it was actually a group of notes played together and no trial and error in the world would get that sound with only one note.
This is where a background in theory would be really helpful. Since then I have taken composition and theory classes and know a bit more on that side of things.
I am not saying that this is how you should do it or the best way/only way whatever. This is just what I did.
To this day some concepts/sounds still confuse me, I am in no way perfect, but this approach to learning sound design has worked nicely so far.
A couple of other things I recommend researching are how digital audio works and the difference between it and analog sound,
maybe learn something about phase cancellation as well.
Lastly, some sounds are very difficult to synthesize (e.g. realistic sounding piano and vocals) because of all the articulation happening, so the only real way to achieve these real sounds is to record them (a whole other rant), so try to spot these and not get caught up in them.
I hope this helped, I tried to keep it eloquent, interesting and to the point.
