Old English inflection survivors

The English language has seen a widespread reduction of inflections over the past 1000 years, but there are a number of survivals—some surprising and others not. For example, the common plural ending for nouns (lasers, malaises, plates) derive from the Old English masculine ending -as, as in cyningas "kings." Similarly, the possessive's -'s ending (as in rocket's) derives from the masculine and neuter genitive ending -es, as in cyninges and scipes.

Below are some others. The last two ask you to do some investigative work in the OED.

willy nilly From an Old English construction using the subjunctive mood of the verb meaning "to wish, will":
wille he, ne wille he
"whether he wants to or not" (lit. "whether he may wish or may not wish")

she works nights nights is not the plural historically but a holdover construction where the genitive was used to indicate increments of time.

go get 'em The 'em is an oral survival of the Old English dative pronoun him, either singular or plural. Contrary to what you may have learned elsewhere, it is not a contraction of them.

ten foot pole The noun foot only looks singular; historically it comes from the OE genitive plural fota, so the older phrase means literally "ten of feet." For similar reasons we might also say "a ten mile walk," where "mile" was once a genitive plural. However, we can still say "I jumped ten feet and ran ten miles," where the nouns have the more usual plurals.

oxen This is the only genuine survival of the weak noun endings, once quite common in Old English, where the plural ending was -an, as in naman "names." Other, near survivors: shoe had shoen alongside the more familiar -s plural for centuries; of Chaucer's Chauntecleer: "Lyk asure were his legges and his toon" (again) alongside the more common toes. The plurals children and brethren are 12th century inventions, where the uncommon -en ending was added to words that already had an uncommon plural (childru, brether). We now use brothers regularly as the plural; the wonder is that childs never caught on.

seldom The more common spelling of this word in OE was seldan, but it was influenced by the dative plural ending -um, which eventually came to be spelled -om. The -um ending (dative plural used adverbially) was once quite common. Chaucer, Spenser, and even later writers like Dryden and Fielding still used whilom from OE hwilum "at times."

the more, the merrier The the in phrases like this one involving a comparative is not the same as in "the dog chased the cat." Instead it's a relic from an old Indo-European case, the Instrumental, which was falling out of use even during the Old English period, but it was retained in such constructions as the one here. The simplest way to describe the case is that it answers the question "by means of what?" Today's unusual the descends from the instrumental case of the OE definite article, spelled Dy. So a comparative phrase like the one above literally means "by means of more, by that much merrier."

amongst Look up this word in the OED. Is the ending a survivals of an older inflection? What about amidst, whilst?

if I were you Is were historically the plural (e.g., "they were old") or a different inflection?

lest, nonetheless What do these two words have in common? And what do they have in common with one of the earlier examples?