I just looked it up. Maple syrup is almost pure sucrose-water syrup. (I know that the colour and flavour are very distinctive, but the molecules responsible for them are a very small percentage by weight). Which means that it behaves just like regular candy crystalization.
The bad news is that candy crystalization isn't easy. There are just too many factors which have an influence on the texture. The good news is that people have already created such recipes. Find such a recipe and follow it exactly to the letter! If it says "wooden spoon", don't use a metal spoon, etc. And don't even start without a thermometer, unless you have had years of experience to perfect your water test technique.
As I haven't ever made or seen this specific candy, I can't tell you which parameter values will give you the needed texture. But I can list the usual parameters which influence the final texture.
As for general understanding, what you have is a saturated sugar solution. When you heat it, water evaporates, making the solution more concentrated, but on the other hand, sugar is more solvable at higher temperatures, meaning that it doesn't get supersaturated when heated gently. When the concentrated syrup cools, it becomes supersaturated, because the concentration doesn't change, but the solvability does. If you give the sugar the smallest chance to form separate largish crystals during cooling, it will do it. For a consistent texture, you must watch following parameters extremely carefully.
the density of your sugar syrup. It is measured indirectly by measuring the boiling temperature of the syrup. The exact temperature at which you must stop boiling will be given in the recipe (don't forget to adjust for altitude, subtract 1°C for every 300 meters). If the recipe only tells you a syrup stage (something like "hardball"), use a chart. Allow leeway for carryover - when you remove a pan from the stove, it continues to heat its contents for a while, so remove two-three degrees early (or more, depending on the pot). You can immerse the pot immeidately in cold water if it has gotten too hot, but this isn't so good for the pot itself.
The speed of hardening. I have a vague memory that you can't use rapid cooling for a non-sandy candy texture, but don't remember it 100% correctly. Rely on your recipe, if it tells you to pour it on a marble slate, do it, or at least use a refrigerated thick metal pan or something similar which cools quickly.
Nucleation points. These are very hard to avoid, but if you need a sandy texture (what you seem to want), it is much easier. If you want to have no nucleation (for a smooth hard candy), you must do everything possible to have no sugar crystals and no impurities in your syrup and to not disturb the pot. This is the reason for most weird requirements in candy recipes, like washing and drying the termometer before each dip, or the abovementioned wooden spoon.
Stove temperature. It must be hotter than your goal temperature, but if too hot, the sugar will scorch on the bottom. Also, the evaporation rate is changed by temperature, which may contribute to supersaturation.
Chemical help. You may need to invert your sugar (use acid, cream of tartar is common - just follow the recipe) or add glucose and/or fructose. This prevents the creation of crystalization nuclei in your supersaturated syrup. You probably don't need that, because it is more important for the popsicle-style candies.
Breaking up crystals. For candies where multiple soft crystals are expected, you may need to manipulate the mass mechanically after the crystalization has started. Kneading and beating are common. They result in the breaking up and mixing the crystals, resulting in a soft mass (fondant, fudge) as opposed to big hard crystals (the ones daniel's method will create). The more you do that, the creamier your texture will get.
As you see, there are way too many variables to experiment with. You could do it, but you'll need lots of luck and patience to hit on the right combination. Pick a recipe, and follow it. If needed, adjust the result according to the guidelines I gave (e. g. kneading more for creaminess).
I just noticed, the site I linked for the chart has a page describing some maple syrup candy types, and lists the correct temperature and basic handling stages for them. Could be a good starting point. (Maple candy page)
I would take some of the maple syrup that you have and cook it down in to granulated Maple Sugar. A tutorial is here. Then, as maple sugar is about twice as sweet as regular sugar, substitute it into your recipe accordingly. That should help infuse your scones with additional maple goodness.
Best Answer
Here is the nerdy, science-y explanation. When you heat sugar + water to a high temperature you are first dissolving all the sugar crystals (maple syrup's sugars are all already dissolved at room temperature), then creating a more and more saturated solution. The higher you take the solution above 100°C, the more saturated the solution gets. Then by cooling without agitation to a lower temperature, you are create a super-saturated solution. A super-saturated solution is an essential step in these types of candies (the same basic technique is used for fudge and fondant as well as maple creams).
At the point where you have the solution cooled down enough for it to be super-saturated, that is when you need to start agitating it. This agitation allows crystals to start forming, but as you guessed in your second question, it also prevents them from getting too large. The precise temperatures used also contribute to the final possible crystal size. This candy is ultimately going to be made of micro-crystals suspended in syrup. If you don't heat it high enough to begin with, the solution won't be saturated enough and the crystals will just dissolve back into the syrup. If you don't cool it enough before starting to beat it, the instability before the solution is super-saturated will cause larger crystals to precipitate out of the solution, causing the graininess (like how rock candy works, but at a much smaller scale).
While I haven't made this recipe before, I suspect that because the end result of this recipe is still liquid, that you could wait until it's even cooler before beating it. I am also pretty certain that if you did cool it for longer, that the whole "beat manually for 20 minutes" step could be replaced with a quick spin in the food processor (if you have one). By comparison, fondant is traditionally made in a similar way by cooling on a marble slab and then agitating it manually. This method that uses a food processor gives identical result (or better, to be honest) with much less physical effort. I suspect your maple butter recipe could have a similar labor-saving update made to it. The one thing you don't want to mess around with is not heating it enough initially, which you've already found by testing, or not cooling it enough before starting to agitate.
Reference: Chocolates and Confections by Peter J. Greweling.