Zodiac vs. Original
"...that nice impriest..." vs. "...that Nisi prius..."
"...judicial hummerest..." vs. "...judicial humorist..."
"...and uncompromising kind..." vs. "...apologetic statesmen of a compromising kind..."

It gets somewhat clear that Z had not read the original text too often. Instead, it sounds as if he had 'learned' the text by listening to e.g. a music teacher. Also, we do know that some of Z's victims had ties to musical education. Also, one should be aware that the Mikado song is not a solo song but comes with a chorus: By nature, the training of such a chorus is rather by listening/practicing after having learned the text/melody once. If text is not learned perfectly, e.g. during preparation of a piece, any chorus member could easily make such mistakes.

Following this thought, Z could very well have been a member of such a Mikado chorus - this not according to his personality but personal interest in such music. Also, it'd be interesting if there had been victims of such chorus.

Certainly, when writing the letter, he did not copy the text from the original printed form but from what he had heard / learned acustically. Also, a record of such song would rather be rare (and not necessarily heard on a daily basis to learn the text from). Thus, I assume he actually prepared for some performance - most likely in a chorus and not necessarily for the role of Ko-Ko, as has been assumed in the past.

By then, the Lamplighter's theater in SF focussed on such Gilbert & Sullivan musicals, the Mikado in particular, which brings up the Lamplighter's connection again: Three months earlier, in fact, the piece was shown by the Lamplighter's at the 'Presentation Theater' (and other locations). Very well Z could still have (still) remembered the text three months later from having been part of the chorus.

Update:
Another little but possibly important detail: Z wrote:

"Ps. The Mt. Diablo Code concerns Radians +# inches along the radians"

In fact, the # (hashtag) symbol very clearly seems to have been added after the sentence had been written without it. Z even points on the hashtag symbol by drawing a small arrow below it. This should not be ignored: The hashtag symbol also represents that the inches are meant to be 'hashed', thus most likely be divided by e.g. ten. "Two point one" radians would therefore be written as "Twentyone". Vallejo, for example, 40 miles away from Mt. Diablo, would be 40 / 6.4 = 6.25 inches or 62.5 ('sixtytwopointfive'). One might conclude that the last letter ended as a number from 1 to 9, thus (ON)E (TW)O etc. thus E, O, R, X, N, T with the second symbol/letter being equal to that; for example the cleartext (assuming it consists of numbers) starting with FOUR... and ending with ...TWO.

Pairs (first, then last number in potential cleartext):

ONE
TWO
THREE
FOUR
FIVE
SIX
SEVEN
EIGHT
NINE

ONE is a match with SEVEN (only).
TWO has no match (as no number ends with 'W')
THREE has no match
FOUR is a match TWO
FIVE has no match
SIX has no match
SEVEN has a match with ONE, THREE, FIVE or NINE
EIGHT has no match
NINE has no match. 

According to this, the cleartext of the map cipher starts / ends with one of the following:

ONE..(..)..SEVEN
FOUR..(..)..TWO
SEVEN..(..)..ONE
SEVEN..(..)..THREE
SEVEN..(..)..FIVE
SEVEN..(..)..NINE

By definition, one radian is 57.29578° degree. Thus, 6.28318 radians make a full 360° degree rose - there is no radian "SEVEN". This rules out all terminology such as "sevenpointthree" in the cleartext. However it lets left e.g. "seventyfive" GRADIANS (1/10th of a radian), but we might ignore that one for the moment. This leads us to either "ONEPOINT..." or "FOURPOINT..." or "RADIAN.." as a potential beginning of the cleartext. This should lead to the direction, with Mag North still to be considered (moreless placing Z into the NE or SW of Mt. Diablo).

Regarding the # inches (or distance from Mt. Diablo):
Staying in a range of let's say 100 miles we are talking about 15.625 inches on the map or a value of maximum 156.25, therefore ending up with (only)

"TWENTYSEVEN"
"THIRTYSEVEN"
"FOURTYSEVEN"
"FIFTYSEVEN"
"SIXTYSEVEN"
"SEVENTYSEVEN"
"EIGHTYSEVEN"
"NINETYSEVEN"
"ONEHUNDREDANDTWENTYSEVEN"
"ONEHUNDREDANDTHIRTYSEVEN"
"ONEHUNDREDANDFOURTYSEVEN"
"ONEHUNDREDTWENTYSEVEN"
"ONEHUNDREDTHIRTYSEVEN"
"ONEHUNDREDFOURTYSEVEN"
"TWENTYTWO"
"THIRTYTWO"
"FOURTYTWO"
"FIFTYTWO"
"SIXTYTWO"
"SEVENTYTWO"
"EIGHTYTWO"
"NINETYTWO"
"ONEHUNDREDANDTWENTYTWO"
"ONEHUNDREDANDTHIRTYTWO"
"ONEHUNDREDANDFOURTYTWO"
"ONEHUNDREDANDFIFTYTWO"
"ONEHUNDREDTWENTYTWO"
"ONEHUNDREDTHIRTYTWO"
"ONEHUNDREDFOURTYTWO"
"ONEHUNDREDFIFTYTWO"

as potential numbers at the end of the cipher. One of those numbers divided by 6.4 actually could give us the distance to Mt. Diablo (of the bomb Z later claimed he had never set-up); e.g. ONEHUNDREDANDTHIRTYTWO >>> 21.406 miles.

Length (letters) of potential radian terminology:

12.onepointzero
11.onepointone
11.onepointtwo
13.onepointthree
12.onepointfour
12.onepointfive
11.onepointsix
13.onepointseven
13.onepointeight
12.onepointnine
13.fourpointzero
12.fourpointone
12.fourpointtwo
14.fourpointthree
13.fourpointfour
13.fourpointfive
12.fourpointsix
14.fourpointseven
14.fourpointeight
13.fourpointnine

With a cipher length of 32 symbols, the radian terminology at the beginning could have between 18 and 21 letters, for example:

21."ONEHUNDREDTWENTYSEVEN"
21."ONEHUNDREDTHIRTYSEVEN"
21."ONEHUNDREDFOURTYSEVEN"
19."ONEHUNDREDTWENTYTWO"
19."ONEHUNDREDTHIRTYTWO"
19."ONEHUNDREDFOURTYTWO"
19."ONEHUNDREDFIFTYTWO"

giving us the direction (radian) 1.1, 1.2, 1.6, 1.3, 1.7, 1.8, 4.0, 4.4, 4.5, 4.9 (...3x3 plus 4x7 = 37 different spots on the map). Sadly I still do not see any (valid) solution for the cipher yet, thus just an idea. So far, the best I can come up with is something like

matching the cipher but the text not really ideal. Opinions?

QT