'The sigh of summer upon my return
Fifteen alike since I was here'
ie - its been fifteen summers (15 years in effect) since he's been to wherever he is in Still Life, each one supposedly 'sighing' in some way.
'The lapse of the moment took it's turn'
This is unclear to me as well. All the terms used here are temporally vague in relation to the rest of the lyrics. The 'moment' seems to be the time up until now, (ie, almost present tense) established in the preceding 3 lines. The moment being his current state of weariness and calm after 'what might have been ages'. The 'lapse' could be the thought of 'Would I prosper or fall', but this is very unclear. The 'lapse' could be something entirely outside the text for all we know. Likewise, the lapse has a 'turn', as if it was waiting to happen, as if after all the ages this moment of contemplation has finally occurred. It might be more for mood-setting than anything else, because there doesn't seem to be specific meaning, other than possibly what I've just suggested.
'Branded a Jonah with fevered blood'
Jonah was a prophet of God but came to reject his preaching duties and defied God. He tried to flee but was arrested boarding a ship because the people sensed he carried a dark and heavy secret. So, in this passage in The Moor, our persona is recalling his banishment from the community those 15 years ago.....the reason is entirely unclear, though it probably has something to do with Melinda, possibly some kind of kinky extra-marital affair or something. So he was branded a Jonah symbolically for carrying this dark secret (obviously a zero-tolerance puritan community) and banished. The fevered blood part can have two meanings, choose your fave. One, it is he who has fevered blood, ie, he's mad and can't help it (in his blood) and the fever being his sexual appetite or whatever else it could be. (this is the more likely meaning given the direct structure of the sentence) or possibly it could be the crowd condemning him that has fevered blood; in a fever of ostrasizing him.
'You'd never leave me to a fate with you'
Well, basically, Melinda on appearances wouldn't allow our persona to end-up with her....for whatever fate may bring. The structure is interesting though, it implies that fate has already chosen our persona's destiny with Melinda, and Melinda is actually by contrast the active agent who must deny this intended fate. This sets an appropriate mood for the album, as it gives us a feeling of inevitability on the part of the narrator, and rests the question and fulfillment of the album on a source we can't be sure of. What will Melinda do? It turns the course of discovery we've been expecting having read the lyrics in The Moor up till then on its head.