J.F.W. Herschel's Cipher Puzzle

I've been interested in ciphers that allow multiple readings (a Venetian example).
So, I took a note when I read about a ciphertext that yields two readings, posted by tonybaloney at:
http://www.aerobushentertainment.com/crypto/index.php?topic=36.60
(now it seems the link has changed).
The ciphertext (A) and the two readings (B,C) are as follows:

(A)Xabnsly ngpwpdetlews tbbbtzl aobl stheingdnxmccvv
(B)Several philosophers observe that chloroplatinate
(C)Sing, Celestial Muse, the destroying

(A)hclzepsf xo qskxybbbbui
(B)solution on silverplate
(C)wrath of Achilles,

(A)Egtubatjkh fba lwipizix eqjbnasv nfvj yjcin
(B)reproduces the luminous spectrum with great
(C)Peleus’ son, what myriad woes it

(A)cjzvekzxy gf nbyr gzrefcwxianst
(B)vividness in blue fluorescences,
(C)heaped on the Grecians,

(A)Jxkivu v xcnukwcxpv ifnnszp't tpdvm
(B)whilst a coppersalt insolated might
(C)Many a valiant hero’s soul

(A)lqaauuqrauaqqvso up mfijtxyz.
(B)photographically be coloured.
(C)dismissing to Hades.

Now, I find this ciphertext was posted by Klaus Schmeh on his blog Cipherbrain back in 2016: Wer knackt die Verschlüsselung des Astronomen John Herschel?. The article shows the source: The Photographic News, 5 January 1866 and identifies the author of the ciphertext as J.F.W. Herschel (1792-1871) (Wikisource). The blog readers found out additional information.
The weekly magazine (Google) carried the puzzle on 5 January 1866 (p.5-6), the solution (B) provided by a reader the next week (p.23), and Herschel's correct solution (C) the next week thereafter (p.35). After all, the reading (B) was wrong, and this cipher is not about double reading.
The readers of Cipherbrain worked out the general principle of this cipher: schorsch pointed out the first two words can be explained by word-by-word Caesar cipher, Norbert demonstrated this works for every word, and Thomas found the shift, counted in reverse direction, is the number of letters in the word plus 1, 2, 3, 4, ....
In the following, (a) is the ciphertext, (b) is the number of letters in the (deciphered!) word plus 1, 2, 3, 4, ..., (c) is the result of reverse-shifting, and (d) is the plaintext word.

(a)Xabnsly ngpwpdetlews tbbbtzl aobl  stheingdnxmccvv
(b)4+1=5   9+2=11       4+3=7   3+4=7 10+5=15
(c)Svwingt cvelestiatlh muuumse thue  desptyroyixnngg
(d)Sing,   Celestial    Muse,   the   destroying

(a)hclzepsf xo    qskxybbbbui
(b)5+6=11   2+7=9 8+8=16
(c)wraotehu of    acuhilllles
(d)wrath    of    Achilles,

(a)Egtubatjkh fba     lwipizix eqjbnasv nfvj    yjcin
(b)6+9=15     3+10=13 4+11=15  6+12=18  4+13=17 2+14=16
(c)Prefmleuvs son     whtatkti myrjviad woes    itmsx
(d)Peleus'    son,    what     myriad   woes    it

(a)cjzvekzxy gf      nbyr    gzrefcwxianst
(b)6+15=21   2+16=18 3+17=20 8+18=26=0
(c)hoeajpecd on      thex    gzrefcwxianst
(d)heaped    on      the     Grecians,

(a)Jxkivu  v       xcnukwcxpv ifnnszp't tpdvm
(b)4+19=23 1+20=21 7+21=28=2  5+22=27=1 4+23=27=1
(c)Manlyx  a       valsiuavnt hemmryo's socul
(d)Many    a       valiant    hero's    soul

(a)lqaauuqrauaqqvso up        mfijtxyz.
(b)10+24=34=8       2+25=27=1 5+26=31=5
(c)dissmmijsmsiinkg to        hadeost
(d)dismissing       to        Hades.

What remains unknown is (i) how the decipherer can know the number of letters in the deciphered word and (ii) how the decipherer can identify the letters to be discarded. The latter may be left to the decipherer's insight, but (i) is indispensable to allow proper deciphering.
Herschel presented this cipher as a challenge to believers of "an axiom that there is no cipher which cannot be read." I guess that in trying to make the puzzle more difficult, he forgot that the scheme need to be (difficult but) invertible.