I'm very late to this, and Bosch is not my particular specialty, but I'll try to address some of your points as best I can.

While we can't get an insight into Bosch's mind, we do know some facts about his life that allow us to speculate on why his works look the way they do. First, he came from a family of painters—not at all unusual for the period. He's first documented as a painter around 1480, and then around June of 1481 he appears to have married Aleyt Goyarts van den Meervenne (d. 1522-23), the daughter of a wealthy local patriciate. Bosch was thus wealthy enough not to have to paint to make a living, which sets him apart from the majority of painters from this period (most of the artists you've heard of from the early modern era worked to make a living). According to tax records from 1502-3 and 1511-12, Bosch was in the top 10% of ’s Hertogenbosch citizens. He was also a member of the Brotherhood of Our Lady, an exclusive social group that also provided his first commissions. It is a sign of Bosch's unusual social status that he was the only artist to be a sworn member of this group.

This independent wealth and high social status may have given Bosch a degree of freedom that other artists lacked. Furthermore, while Bosch is among the most distinctive of 16th-century painters, he was still working within an established tradition. For example, he painted at least sixteen triptychs, a well-established format in Netherlandish art, although his approach to this format could be remarkably subversive and inventive (e.g., making the exterior as important as the interior or unifying the imagery across all three panels). The Triptych of the Hay Wain is an example of how inventive he could be within an established form.

Bosch was certainly known and collected in his own day and in the decades following his death. In the first half of the sixteenth century collectors included Philip the Fair (Duke of Burgundy and King of Castile), Isabella the Catholic (Queen of Castile), Margaret of Austria, and Philip of Burgundy, among others. There was particular interest in his work in Spain, and he remained popular in art historical writings there until about 1800. He was still mentioned in writings outside Spain during the 17th and 18th centuries, but generally without much interest. This changes at the end of the 19th century, when interest is renewed. I've read that more iconographic studies are devoted to Bosch than any other Netherlandish artist, and while I haven't counted to confirm this I would not be at all surprised.

As for his technique, it appears to have differed across his oeuvre. He apparently used several different types of underdrawing techniques—in some instances sketching just a general design with a coarse brush and diluted paint, which meant many changes during the painting process; in other cases using extensive hatching (shading with fine parallel lines), suggesting a greater interest in accuracy and less changes during the actual painting. Bosch also used different pictorial styles, sometimes painting very carefully with thick layers of impasto and sometimes applying paint more haphazardly in very thin layers (and just about everything in between). The left wing and central panel of the Garden of Earthly Delights is an example of the former style, while The Hay Wain is an example of the latter.

While Bosch is distinctive, there are other artists whose works show his influence; Joachim Patinir and Quinten Massys are two examples. We also have some evidence that he was deliberately copied by artists trying to pass of their works as his. The 16th-century connoisseur and critic Felipe de Guevara wrote the following in his "Commentarios de la Pintura” of c. 1560:

That which Hieronymous Bosch did with wisdom and decorum others did, and still do, without any discretion and good judgment; for having seen in Flanders how well received was this kind of painting by Hieronymous Bosch, they decided to imitate it and painted monsters and various imaginary subjects, thus giving to understand that in this alone consisted the imitation of Bosch.
In this way came into being countless numbers of paintings of this kind which are signed with the name of Hieronmyus Bosch but are in fact fraudulently inscribed: pictures to which he would never have thought of putting his hand but which are in reality the work of smoke and the short-sighted fools who smoked them in fireplaces in order to lend them credibility and an aged look.
I dare to maintain that Bosch never in his life painted anything unnatural, except in terms of Hell or Purgatory as I have mentioned before.

Of course, we need to read this with a critical eye: in his fervor to prevent wrongful attributions Felipe de Guevara may have gone too far with "disattributing" works (something which also likely happened when Bosch was "rediscovered" in the 19th/20th centuries).

All this is a roundabout way of saying that while Bosch is certainly distinctive, and while he certainly resists easy interpretation, there is a wealth of material on him. I'm drawing from my own teaching notes (which may not reflect the most recent research, so caveat lector), much of which was probably based on Grove, and a couple articles here (Lynn F. Jacobs, "The Triptychs of Hieronymus Bosch" The Sixteenth Century Journal 31, Winter 2000; Larry Silver, "God in the Details: Bosch and Judgment(s)" The Art Bulletin Dec. 2001). You might try something like Craig Harbison's The Mirror of the Artist: Northern Renaissance Art in its Historical Context for a good introduction to Northern Art broadly (with some specifics on Bosch).