[Legate of Amon Lanc]

Originally Posted by Gordis

And I think you are overrating him. Sure he cut an impressive figure on his monstrous horse at the parley - but was he really THAT great?

I'm sure he was (see below).

Was he entitled to give advice to Sauron? I doubt it.

Not sure, but at least:

Originally Posted by The Black Gate Opens

and knew much of the mind of Sauron

Which implies something like that. Not that Sauron is one who takes advice from anyone, but this one had probably the closest to it.

Let me catch you on your own words:

And "Lieutenant" here is not a definite army rank (like nowadays) but has the same meaning as second-in-command.

Second in command, indeed! Now see this (about Mouth of Sauron):

Originally Posted by The Black Gate Opens

The Lieutenant of the Tower of Barad-dûr he was

Read what's written there about MoS - there was even debate here back then whether he couldn't have been of higher rank than WK himself (nonsense in my opinion, however not that overstretched).

Also, Sauron didn't hesitate to send MOS to the parley, protected only by ambassador's diplomatic immunity. Could Sauron be 100 % sure that the Lords of Gondor wouldn't chop off MOS's head, like Aragorn did in the infamous PJ's scene? Sauron was treacherous and judged others by his standards - MOS himself was much afraid. That's why I call him expendable.

There's ambiguity in this. For Sauron, everyone was expandable, for ultimately, he did not care for anyone but himself. However, as long as he had some servants who were good for him, he cared about them more than about the others - and MoS clearly belongs in this cathegory. And the scene with diplomatic immunity (MoS's words that he can't be attacked) shows quite the opposite than you suggest to me: he counted upon the diplomatic immunity, do not say "only by", because the codex of honour and all these things were valued in Middle-Earth (cf. Gandalf's response). It was not a sort of custom that can be broken wilfully, no, indeed, only the "low master of treachery" would do that.

And let us consider his dream - to become the new Lord of Isengard. Was this position better than Sauron's right-hand man, his Prime Minister?

Not sure. But he would be like the lord of his own realm, indeed, all the West would be his, and far from Sauron himself, he will be almost free to do whatever he wants (or at least, he naively thought so). Prefect of a large but distant province.

Nay, but he made sure that everything ran smoothly, was in order, including the latrines. If there were a problem of this genre, do you think the orcs would report to Sauron? Nay, they had MOS for it at the top of a long chain of command.

No, they told that to the fifth deputy to the eighth advisor to Mouth of Sauron. That's why the chain of command is there.

I am sure that Gothmog was the Lieutenant of Morgul (Lord)=the WK - not Minas Morgul fortress.

That's more or less the same, in my opinion.

I believe the nazgul were under the WK's direct orders, not Sauron's. Every army needs a clear chain of command from top to bottom - it is never good when someone other interferes (even Sauron).

Yes, but the root of my idea is that the Nazgul were not there as part of the army. They had nothing to do with the attack on Minas Tirith. They were there as "outside force", to mess around and eventually carry messages and scare Faramirs.