In 18th century England, was "eat" the past tense of "eat" and how was it pronounced?

Yes, eat (pronounced /it/ and possibly /ɛt/) was a past tense form of "eat" in English until the 1800s and more recently in Scotland.

The OED lists a range of past tense forms of eat. The relevant ones are

1500s–1600s eate, 1600s–1800s eat, 1500s– ate

So "eat" (/it/) was a past tense form of "eat" in Boswell's time, although "ate" was also possible.

The OED also suggests the non-standard past-tense form "et" (/ɛt/) may relate to "eat" rather than "ate", by analogy with other past tense forms "read" (/rɛt/), "led" (from "lead"), etc.

The OED does not give a geographical range for the variants. The Scottish National Dictionary has citations of the /it/ form as late as the 20th century, including from Aberdeen in 1925

Peter, my neeper, mairriet a wife An' cudna keep 'er, He stappit her intull a bole i' the wa', An the mice eet 'er.

This uses the spelling "eet" rather than "eat" but spelling in Scots is considerably more varied.

James Boswell was of course a Scot, born in Edinburgh.

• "eat, v.". OED Online. March 2023. Oxford University Press. Accessed 22 May 2023. https://www-oed-com.nls.idm.oclc.org/view/Entry/59131
• "Eat v., n.". Dictionary of the Scots Language. 2004. Scottish Language Dictionaries Ltd. Accessed 22 May 2023. http://www.dsl.ac.uk/entry/snd/eat

Yes. In addition to the answer citing the Oxford English Dictionary, Johnson's Dictionary of the English Language (1755) also lists both ate and eat as the preterite (past tense) form of eat:

To EAT. v.a. preterite ate, or eat; part. eat, or eaten. [etan, Sax. itan, Gothick; eich, Erse.]

As for pronunciation, in John Walker's A Critical Pronouncing Dictionary and Expositor of the English Language (1791) (found in Eighteenth Century Collections Online; subscription needed), he claims that to eat is pronounced with a long /i/ but that the past tense form is usually pronounced et (/ɛt/):

The first sound of ea is like open e, and is heard in the following words: - Afeard, anneal, appeal, appear [...] easy, to eat, eaten, eaves [...] (p. 28)

The preterimperfect tense of eat is sometimes written ate, particularly by Lord Bolingbroke, and frequently, and, perhaps, more correctly, pronounced et, especially in Ireland; but eaten always preserved the ea long. (p. 29)

Here in Yorkshire /ˈɛt/ is commonly used in Yorkshire dialect. And the past participle is /ˈɛtn/ (I think I've transcribed that correctly). Even people who don't speak dialect usually sing the last but one verse of the song "on Ilkley Moor baht 'at" using /ˈɛtn/, usually written "etten". (Then we shall all have etten thee...)