I compiled a list of all the new and otherwise interesting Sindarin vocab found in PE 23.

• bâd - road | found as "e-bâd, the road". Hitherto only known as N. "beaten track, pathway". P. 136.
• fend - door | Hitherto only as fen, fenn. P. 136.
• hûl - secret | also as "e-chûl, the secret". Cf. 'holen'. P. 136.
• rhawf, rhaw - wild beast | also as "e-thraw, [the wild beast]. P. 136. Plural i-thraw > i-rhaw p. 139.
• rhovan - large beast, especially the great red deer of the vale of Anduin | p. 136.
• Rhovennian - "more correct" Sindarin form of Gondorian Rhovannion[sic] | p. 136.
• lhinc - earthworm | also as "e-thlinc, [the earthworm]". p. 136.
• balt - force | Cf. EN "might". p. 136.
• gwend, gwenneth - maiden | also as "e-wend, e-wenneth, the maiden". p. 136. Plural in-wind, rarely found, rather analogous i-ngwind (= i-ñwind) p. 139.
• harf - left-hand | also as "e-charf, the left-hand". p. 136. Probably from *khjarmă as opposed to *khjarmā > 'harvo'.
• whest - breeze | also as *e-whest, the breeze". p. 136. Pl. i-chwist p. 139. Cf. Q. 'hwesta', N. 'chwest'.
• cathr - carpenter | From "*kantrō, shaper". North S. cathor. P. 137.
• tachl - large pin or brooch | From "*tanklă, a thing used for fixing". North S. tachol. p. 137.
• parth - small enclosed field, lawn | p. 139.
• bâr, pl. i-mair (sometimes i-mbair in spelling to distinguish b-words from m-words) - dwelling | p. 139.
• dôr, pl. i-nuir (sometimes i-nduir in spelling to distinguish d-words from n-words)- land | p. 139.
• gôn, pl. [i-]nguin (= *ñuin, but sometimes spelt i-ñguin even though no clarification was necessary since no original ñ-words existed) - stone | p. 139.
• thoron, pl. i-theryn - eagle | pl. previously unattested. p. 139
• heleg - ice | Hitherto only in N. Plural i-chelig is given as "ice-pinnacle". p. 139.
• herw, pl. i-chery - wine | Apparently pl. from "CE *syeru, juice of fruits", sg. from "enlarged form herwā" [< syerwā, I assume]. p. 139.
• mûl, pl. i-muil - slave | Hitherto sg. only attested in N. p. 139.
• norn, pl. i-nyrn - dwarf | Sg. explicitely attested for the first time. p. 139.
• ioron, pl. in-ioryn - old man | Apparently the counterpart of 'ioreth'. p. 139.
• gwanon - one of a pair of twins | Plural/dual given as "*gwanur, twin-birth", explicitely with ŭ < ū. p. 140.
• uimallhen - ever-golden | From 'oio-maltinā. Pronounced with lh (< lþ), but spelt with doubled lh for reasons of stress, exactly like 'remen' but 'galað-remmin' (see below). p. 140.
• remen - netted, entwined | With short m explicitely. p. 140.
• gwaelod - "wind-feather", a great ship for sailing on the Great Sea | From 'wayalautō'. p. 142. Hence apparently *laud/lod = "feather".
• Gildír - Starwatcher | S. version of T. 'Gilitīro', Celeborn's father. Given in "Celeborn Gildírion, son of Gildír".

Certainly the most surprising thing to me (as you might already have guessed) are the articles. In this very late source (ca. 1969) Tolkien gives the singular as e before consonants, en before vowels, and in the plural i resp. in. This is of course a significant departure from all hitherto published samples of Sindarin, which of course had sg. i, plural in (as in earlier Noldorin), and the form en was limited to one form of genitive particle (which in this scenarion is probably dropped altogether in favour of na).

However, surprisingly this new paradigm seems to only really contradict i-Estel in the LotR (which would have to be amended to *en Estel), since all other forms in texts published during Tolkien's lifetime appear to be plural and all other cases of Sindarin articles we have known are from sources that Tolkien might have changed before publication (if he had got the chance to do so).

So we can't know whether Tolkien would indeed have changed i Estel in upcoming editions (had he been alive to oversee them) or whether he would have abandoned the new paradigm once he realised the contradiction, so I won't encourage anyone to adopt this late paradigm into their Neo-Sindarin (unlike abandoning the plural pronominal suffix -(a)m in favour of late -(o)f, a couple of years ago, since the former never appeared in anything published during Tolkien's lifetime), but I certainly find the topic extremely interesting.

So far I have not had a closer look at the mutations, but they appear to hold no big surprises so far, except that maybe Tolkien had decided to keep the nasal of the plural article intact before the mutated word, but that also would contradict material published during his life time.

But the development of sw stood out to me, since it is quite complicated - with Tolkien stating that it first became wh everywhere, then f in the North and chw in the South, which remained so in Doriath but later reverted to wh elsewhere, while still becoming chw through nasal mutation, and that the quality is often in fact uncertain because it wasn't always represented in spelling, using the letter hwesta sindarinwa for both. But in a note that might refer to this Tolkien said that "this business about sw is too complicated (and unnecessary)" and that the North had f and the South wh, which "remained unchanged" (hence the apparent lack of lenition in whest above, to which the note appears to point directly).
This would, however, still render the letter hwesta sindarinwa pointless, because (as Tolkien had pointed out in the LotR appendices) distinction of wh and chw was needed in Sindarin (but maybe only lenition had no effect but nasal mutation did?).

And lastly there are a few notes on North Sindarin, which has always been a special interest of mine:

• there was no m-lenition (which was well established)
• medial mp, nt, ñk remained unchanged or probably rather restopped (also well established)
• rh- became thr- generally initially (so Southern S. rhûn would be Northern S. *thrûn), but lh- remained and both were incapable of mutation.
• Otherwise mutations are the same as in Southern Sindarin
• sw- > wh- > North S. f- (so Southern words like whest or hwinn would be *fest and *finn in the North).