Saruman was not dominated by Sauron through the Palantir. He, like Denethor, thought that he could use it despite Sauron's control of his own stone. Saruman's plans were and always had been to get the Ring for himself, which meant breeding his own orcs and pretending (at least from his perspective) to cooperate with Sauron.
As far as the Dernhelm issue, I'd argue that anyone could have picked up Merry (but Eowyn understood his desire not to be left behind so it makes sense for it to be her). At this point he had had minimal contact with her, which also explains why he didn't recognize her. And Merry does try to remain secret anyway. People have also theorized that Elfhelm knew who Dernhelm was.
The company waited to set out from Rivendell because they didn't know how safe it would be and they certainly didn't know what had happened to the Nine. Kind of stupid to walk out on your quest and then run into the Nazgul all over again... And they were clearly burned out by the time they reached Lorien. The hasty stroke often goes awry and if they had just stopped there long enough to rest physically they would have broken down much sooner.
Where does it say that Boromir's horse returned to Rohan? For some reason I always read "lost" as "the horse drowned" but I haven't reread that portion of the books for a while.
Two of your queries I'm afraid I can only answer well through non-scholarly works.
As for Gimli hewing naught but wood, I give you some
pedantry.
And as for Gwaihir,
who's to say that the lofty Eagles are without temptation?
And with that, I'd like to offer up an inconsistency of my own:
On the door to Moria, Moria is referred to as Moria, even though it was never given that name until Durin's Bane was awakened. Huh?
Unfortunately at some levels you can see the stagecraft. But you have to be looking pretty closely if you ask me.