Snapedom

Regarding Voldemort's recruitment techniques in the first war: I think we have some evidence of him tayloring his methods individually to his audience: Barty Crouch Jr was attracted to another son of a most disappointing father, Quirrell (OK, that was after the war) was convinced by philosophical argument about the meaning of good and evil (which is why I think he must have been a Ravenclaw). Voldemort knew what would motivate each individual, where the person's handle was.

I agree that by his re-embodiment he had lost his touch. Notice that all the way until he took over the Ministry he hardly did any recruiting. On the day Harry left 4PD, when surely Voldemort would want as many people as possible in the sky all he can muster is thirtysome people, about the same number he had in the circle at the end of GOF. And in GOF there were still 10 people in Azkaban and one late-comer (Severus). So OK, Peter was guarding Ollivander, the wandless Lucius must have been left at home and perhaps there was a team headed by Yaxley doing some work at the Ministry. But there couldn't have been that many DEs. I don't think there were any full recruits from the end of GOF until the fall of the Ministry besides Draco. I suppose Stan Shunpike (and maybe the two others who were arrested by Scrimgeour for suspected DE association) were held in Azkaban in the same area where the genuine DEs were so they were all broken out together and forced to join, but we do not see Stan with the DEs later on, I doubt he was a fulltime recruit. After the Ministry was taken then perhaps there were people joining the bandwagon. Voldemort indicates that some of the Slytherin students joined the fighting on the DE side during the battle of Hogwarts. But despite the expectations of many fanfic writers, we do not see recent Slytherin alumni, not even Montague, let alone recent alumni of other Hogwarts Houses, recruited by Voldemort.

As for the difference between a full Marked DE and hanger-ons like Fenrir Greyback, I'm not sure how impressive one had to be considering that the Carrows got their Marks. I would tend to think the criteria were generally rather loose, but someone like Greyback couldn't be invited in because he was a part-human, and a werewolf at that. (I don't think Voldemort would want to be forced to avoid calling everyone to a circle on full moons.)

Severus' position during the first war: I think the first indication that he held special status was that he was Voldemort's choice as his spy and DADA teacher (and also very likely to be targeted for death-by-DADA-curse). That's assuming Trelawney was correct and Severus indeed was going to be interviewed after her and not merely hanging around at the Hog's Head. If he only became Voldemort's appointed spy later on then it was the prophecy that really brought him to Voldemort's attention.

Voldemort's promise to spare Lily: I think the promise was part of a manipulation by Voldemort to send Severus to Dumbledore. The only reason the promise needed to be made was that Severus found out that the Potters were going to be a specific target because of the prophecy, and how else would Severus have found out other than that Voldemort told him? Voldemort went alone to Godric's Hollow, nobody but Peter was involved at all, and nobody in the DE camp but Voldemort and Severus knew about the prophecy before Voldemort's return.

Severus' explanations to Bellatrix: He knew she'd be trying to catch him at a lie, so whatever he said had to agree with what she'd find if she asked other DEs, and even Voldemort himself. So I think he did have a hand, however remote, in Emmeline Vance's capture, and he had at least tacit approval from Dumbledore for that, if not more than that, considering Dumbledore's tactics at other times (for example Harry's removal from 4PD).