I'm trying to find a prominent "Urquhart" who would've written about the American Civil War, and I'm coming up blank.

I'm going to boldly state no, no historians seriously argue that Russia "prompted" the US Civil War, let alone to undercut an agricultural competitor.

The government of Tsar Alexander II, undertaking reforms such as emancipating the serfs, saw similarities in outlook and goals with Lincoln's Republican government. Both the US and Russia were wary at the time of the role the UK and France might play in the American Civil War (and in great power politics as a whole).

As a result, the Russian Foreign Ministry came out in 1862 with strong support for American unity and for Lincoln's government, and sent squadrons of six warships each on goodwill visits to New York and San Francisco in 1863 (you might see much online about Russia "saving" the union in the civil war, but this is an exaggeration in the other direction).

One American agricultural product that DID influence Russia was cotton. The worldwide drop in supplies of (Southern) American during the war prompted countries such as Britain and Russia to look for new producing regions to provide a new supply. This led to greater involvement of Britain in Egypt, and to the Russian conquest of Central Asia and the development of large scale cotton agriculture in both places. In the case of Russia, Central Asian cotton was used for the domestic Russian textile industry; it wasn't competing on international markets for a share of global trade.

A decent history of US foreign relations that touches upon US - Russian relations in the 19th century is George Herring' s From Colony to Superpower: US Foreign Relations Since 1776