I would suggest that Gandalf would indeed have been able to see Frodo. I've nothing to support that of course, for such a situation (Gandalf trying to see an invisible Frodo that is) never happens. The closest being to Gandalf that experiences such a thing must be Tom Bombadil. If he could see Frodo then so could Gandalf the White (before anyone says it, I am not making a comparison between Gandalf's power and Tom's. OK).

The fact that Gandalf also possessed a Ring of Power may also have aided Gandalf in such a detection, but I don't think he would have needed the help in the first place.

: What happens to Sam?

If Gandalf arrived early enough, the he would have saved Sam. This would not have been a problem since he was close at hand and there was nothing there that Gandalf the White couldn't have handled.

If Gandalf didn't arrive earlier enough then things may have gone badly. If Frodo had got to Sam, Sam's life would have been in danger, if the Nazgul had got to Sam then he would be dead.

: Could Gandalf, now the White Rider, get the ring from Frodo (with or without the finger!) and could he then himself have thrown the ring into the fire?

I don't believe there is ANY doubt that Gandalf could have taken the ring from Frodo, as others here have mentioned. Gandalf was a Maia, and an enhanced one at that. Frodo was a Hobbit with no personal power at all. No matter what you multiply zero by, you still get zero. Frodo had no power to enhance in the manner that Elves and greater beings had. Gandalf would, assuming he could see him, have swatted him with ease.

Gandalf would also have been able to lob the Ring into the fire. I suppose one could suggest that having refused to touch the ring in the past could mean that Gandalf would refuse again and get Sam to do it, while learing over his shoulder, but I think not. In the Gandalf the White incarnation I don't believe that the Ring was a temptation for Gandalf anymore. In his possess the One would not have lasted long as it arced gracefully towards the awaiting lava.

: If he had to kill Frodo, would he have?

Yes.

: Could Frodo command Sam?

Well I know that I said above that stuff about zero above...but! I suppose that I may have been a bit harsh on the poor creatures (hobbits that is). OK say they had a tiny amount of personal power, or rather let us say Frodo did. He was after all a Baggins and related to the Tooks, so he was the cream of hobbit society. This taken as said, for the moment, I suppose that he may have been able to Command Sam, but I still find it unlikely. After all he couldn't Command Gollum at the end. Although, of course, he did say that bit (several chapters earlier) about Gollum casting himself into the fire and stuff, which came true, but that I think was more a coincidence than anything else, and at that point the only way to ensure the Rings destruction.

: Could Frodo command Gandalf or the Nazgul?

Command????? Gandalf????? No chance!

Command the Nazgul? Slightly more chance, but still no cigar! Only Sauron could command the Nazgul. Only the Master of the One Ring. OK, if one of the wise had taken up the ring and Mastered it, they would have had obedience from the Nazgul, but that didn't happen. Only the master of the One could command the Nazgul and no one who ever had it had that sort of power and those who did or may have been able to do it, never possessed it.

Good little chat! Cheers!

Padster