Each cleric must have his or her own deity
As I quoted upthread, in the PHB it says that a cleric "is dedicated to a deity, or deities" (p 20) though later on (p 40) we are told that "the cleric might well be judged by his or her deity". I think there is some ambiguity over whether a cleric must have a
single god. (As is the case, say, in 4e and is thus one of the points of contrast between clerics and invokers.)
I realise this is a bit orthogonal to the point of your post. But I think it reinforces that Gygax really was making some of this stuff up as he went along, and wasn't presenting anything as systematised as found in, say, 4e or even Planescape.
In Q1 the lesser god abilities were optional, yes. As in the DM had the option to EXCLUDE them.
"As a lesser goddess, Lolth has certain attributes common to all divine beings. The DM may choose not to use these in this module, since a properly-played Lolth will easily destroy most invaders."
I'm not sure what your point is. It seems clear to me that the treatment of these abilities as optional indicates that it was not thought that Lolth granting spells to her clerics depended upon her being treated as a lesser goddess in the DDG sense of that term.
That doesn't mean that they were granted spells, though. You could have a land of people that worship the great rock. It's just a rock that sits in the center of the capitol city and is attributed(falsely) with all kinds of miracles. Nor does it mean that they all worship devils. There could be some that also worship Hextor and those are the ones who are clerics. "A land of devil-worshippers." doesn't tell you much when it comes to whether fiends and gods are the same or close to the same.
Well, whoever wrote the Hierarch scenario for the City of GH thought they had clerics, of Asmodeus, who could use spells up to 7th level. So I'm obviously not the only person to have interpreted the Horned Society in the way that I always have!
I can't find this in the 1e MM
You mean you don't have it? I provided the quotes and references upthread. They're found on p 84:
The social structure of the sahuogin is based upon rule by a king who holds court in a vast undersea city deep beneath the waves. This overlord's domain is divided into 9 provinces, each ruled by a prince. Each of these nobles controls the small groups of sahuagin dwelling in his fief. This organization mirrors that of the planes of Hell, for the sahuagin are devil worshippers. . . .
[T]here is a 10% chance per 10 male sahuagin that there will be a cleric (evil) and 1-4 assistant priestesses, for the religious life of these creatures is dominated by the females. If a cleric is with the group in the lair, she will be of 5th to 8th level ability, and her lesser clerics will be 3rd or 4th level.
Here we have clerics, and (religious) worshippers, of devils. It's pretty clear-cut.
Sahuagin date back to the Blackmoor supplement, but in that book they have MUs rather than clerics, and their origin is described thus:
it is said that the sea elves and mermen were created by the Great Gods of Neutrality and Law while the Gods of Chaos bent their will to create the Sahuagin. In every aspect the Evil ones sought to make the Sahuagin into the most evil of the evil and many agree that they succeeded . . .
Blackmoor has no discussion of any religious life of the Sahuagin.
To the best of my knowledge Sekolah is original to DDG, ie some years later than the MM which introduces the religious and devil-worshipping elements.
