Learn English – Can any noun ending in -ism be swapped for -ist

I think your underlying premise, that -ar is a variant of -al as far as English is concerned, is not completely accurate or how these suffixes are perceived in presentday English.

In previous stages of Romance languages, speakers may have perceived the two as variants of one another, but I'm not sure this is the case today. The suffix -al is used either to form an adjective from a noun or to form a noun from a verb, whereas -ar primarily is an ending on adjectives, without the corresponding noun necessarily existing in the first place.

Interesting cases to consider include "peculiar", "lunar", "nuclear", "vulgar"...

Your -st endings are two different grammatical animals. In wouldst it is the standard verb ending for the archaic second person singular familiar thou:

Cleopatra. O, I would thou didst,
So half my Egypt were submerged and made
A cistern for scaled snakes! Go, get thee hence:
Hadst thou Narcissus in thy face, to me
Thou wouldst appear most ugly. He is married? — Antony and Cleopatra 2.5.1174–8.

Unbeknownst, however, is another creature entirely, even among its peers.

Emerging in both British and North American print sources in the 1830’s, unbeknownst was originally a colloquialism coined on the pattern of much older words such as unawares (1530s) or always (early 13th c.), adding the etymologically intrusive final t of amongst, whilst, betwixt, etc. The s is a remnant of the genitive case, often used in Old and Middle English to form adverbs from nouns or adjectives.

I think that 'ere Ingian must have been the devil, or how could he come so sudden and unbeknownst upon me, … John Richardson, Wacousta, or, The Prophecy: A Tale of the Canadas, London, 1832.

“I remember my respectable first-floor, Monsieur Boncoeur, bringing home a piping bullfinch last year he’d bought on the Boulevards, whose red breast washed off the first showery day, all as one as Ma’mselle Isoline’s rouge after a flood of tears in a melodrame! The poor dear gentleman had half a mind to have up the seller of the impositious bird before the commissary of the district; only, as he’d paid for it with an old coat unbeknownst to his valet, and an old coat not being lawful coin of the realm, there was a doubt in his mind about his power of bringing the vagabond to justice.” — Toby Allspy (pseud.), “Adventures in Paris: The Five Floors,” Bentley’s Miscellany 2 (1837), 501.

“No matter,” added Lynx authoritatively ; “getting into another man’s barrel unbeknownst to him in the night-time, is burglary.” — Joseph C. Neal, Charcoal Sketches; or, Scenes in a Metropolis, Philadelphia, 1839.

The word also appears in an 1875 Sussex dialect dictionary, suggesting the usage is older than its first appearance in print:

Unbeknownst. Unknown.
“All I can say is, if he comes here, it's quite unbeknownst to me.” — William Douglas Parish, A Dictionary of the Sussex Dialect and Collection of Provincialisms, Lewes, 1875, 127

Even so, the word is a likely candidate for the last coinage in English using the adverbial genitive.

The adverbial genitive, though no longer productive, is a feature especially common to West Germanic languages. It’s hiding in plain sight in many common English words: once, twice, sideways, backwards, forwards and the alternatives amongst, whilst more common to British usage.

This means that you can no longer form words using the adverbial genitive -s, and unless you’re in a Shakespearean play, you won’t be using the thou form of English verbs anytime soon.