I see going down Spinx with light weight as not a problem. The problem arises with the heavier packs. I normally head into the gulf from Pinkham and then base camp. This allows me to go super light when climbing and allows me to do alot more miles. I have also base camped at the perch and have done the ridge from that side the same way. Your trip that you have planned has alot of elevation gain and loss. With full packs you are going to be pushing yourself pretty hard. If you are looking for a challange I think you will find it. All trails out of the great gulf are amazing. My favorite is six husbands but I would not suggest descending it. I have always taken it up to the ridge but never come down it. My other favorite is Wamsuttea. Your trip sounds like alot of fun so enjoy.
From your response - that you are very in shape, experienced hikers - and that you obviously understand the concerns that have been raised here - you folks should have ABSOLUTELY NO problem with ANY of the proposed routes you are looking at. Everyone agrees that there are probably NO easy routes up to the top of the ridgeline - no matter WHICH trail you take up. Mount Washington is a struggle - a challenge. Let's face it - that's why we're all there!! To reach the summit is the end of the fun. All of these trails that've been mentioned - most of which I've been on - are all gorgeous hiking trails - each with their own unique perspective - and each with their own unique challenges. All I can say is - I'm jealous. I could never spend enough time hiking in the Whites . . .
Please write back and let us all know your thoughts - and what route you eventually go with.