Building an addition to my truss-roofed asphalt shingled roof

When tying a new roof into a old roof, some say they just nail the valley board directly over the old shingles and others say best to remove the shingles then nail down the valley board. The roof pitch is 6/12 and adding a sunroom that is 10’ x 16’ with the 10’ along ridge length (not counting the tie-in ridge of 4’6") x 18’ truss length with a 3/12 pitch.

Also I have the original truss specs. 20 PSF additional BC live load check per BOCA96. States deflection meets L/360 live and L/240 total load.

Questions: if no rain in forecast, should I remove the required shingles or just nail over existing singles. Actually I’m thinking to cut out a hole to allow ventilation from old into new?
And assuming my trusses will support the addition of the tie-in, as the sunroom walls will support the rest of the roof?

I would strip the shingles where the tie in is intended and as a temporary measure nail some synthetic underlayment in their place.
I have redone lots where they simply nailed the valley rafter through the shingles, it never comes out as clean as removing.

Tying the attics in together is a solid plan as well.

The roof has laminated shingles, Certainteed, I should still have a bundle left I could look in see the exact shingle. Not sure the best plan for this. Once I get the framing done and trusses in place. I need to mark out where the new roof ridge will met to old roof. Then chalk a line for the valley. Then cut inside that valley at least 12" = cut through the singles. Then strip out these shingles and cut my vent hole. Take up the nails from the old roof back far enough so that I can apply a membrane. Or should I go with a open valley?
For a closed valley after framing the new roof in and covering with osb, I need a plan to make sure this valley dont leak. So I should apply a membrane, whats the best kind here? Then felt 30lb.? Then build the new roof first, then nail down the old roof and then chalk and cut off the shingles up to the valley?
If it wasnt for trying to miss a vent pipe I may have gone with a 4/12 instead of a 3/12. Plus less singles to take up on the old roof. I live at the edge of the forest in NC, so leaves and such is always a problem in the valleys.