The first question to consider is who actually owned keyboard instruments in this period. That raises two sub-questions: (1) the cost of instruments compared with typical wages, and (2) whether people were living in conditions where owning and using an instrument was practical.
I don't have any detailed answers, but one piece of data from London in 1804 shows that the cost of a new keyboard instrument was comparable to the total income of a piano teacher for 6 months. Today in the developed world, the cheapest keyboards cost only a few days' income for somebody on the average wage - i.e. anybody who "really wants a keyboard" for themselves or their children can find a way to buy one. If the cheapest keyboards available today cost say £10,000 (an equivalent sum relative to modern wages of the situation in 1800), that would be a very different scenario.
If one assumes therefore that most "musical amateurs" were from the upper classes or aristocracy, you need to consider how they would have been educated. The basic structure of education followed Plato's guidelines, and the seven liberal arts subjects were divided into three elementary ones (named the "trivium" in Latin) studied first, followed by the remaining four (the "quadrivium") which included music. Upper class children would therefore all study music as a compulsory subject perhaps starting at age 10 or 11, and most likely with daily one-to-one lessons from a tutor rather than in a "school class". That was Scarlatti's "day job" at the Spanish court, for example - the 500+ sonatas he composed were written as exercises for his pupils, (or more accurately pupil, singular) not as some grand artistic endeavour.
Given that scenario, it seems plausible that anybody who owned a keyboard instrument was likely to be competent at playing it, and also at composing - those with no aptitude or interest were unlikely to be "dabbling" with the equivalent of "Fur Elise".
These assumptions seem to be consistent with anecdotes of the interactions of professional composers and performers with their "amateur" patrons - e.g. Goldberg (plus an assortment of the numerous German aristocracy) and Bach, or the various patrons of Beethoven who had large-scale solo piano works dedicated to them.
To take another example, Haydn spend most of his working life (before he became an international celebrity) with the "official" post of a servant in the household of a prince - but that prince funded his own private orchestra, and built his own opera house! Clearly with that scale of expenditure on music, the cost of a keyboard instrument would not have been very significant.
In the 19th century, things began to change - in particular, the majority of amateur keyboard players (perhaps as many as 80%) were women who relied on their husband's income. But that is outside the OP's time frame...