Roofing Advice Needed

I have a house with a small roof leak problem. You can see photos and a detailed description of the problem at http://www.turbo210.com/badroof
In summary:
The house was built just over 10 years ago in the Southwestern United States. It has a relatively complex, pitch roof roof design, with concrete tiles over roofing felt. For several years a small amount of water damage was noted on the interior drywall ceiling and down one wall near the front entrance. This corresponded to an area of the home that was under a separate peak from the main house. Visual inspection of the roof did not identify any obvious problems, other than an accumulation of birdnest debris under some of the tile. Hosing the roof down with water in that area also was unrevealing. Access to the attic immediately under that area of the roof was not possible as it was separate from the main attic.
The leak continued, so about 3 months ago, a roof repair company was called in and requested to identify and repair the leak. The supervisor inspected the roof, confirmed that there was no deterioration of the underlying felt, and no obvious entry for the water in that area. He ended up slopping a considerable amount of tar around on the seams and joints in the area. That seemed to fix the problem for a few months, but water drops showed up in the interior during the next major rain.
Cutting an access point into the sealed attic and inspecting that area of the roof deck from underneath revealed a couple of pinhole leaks in the OSB deck as well as water coming through a seam. Removing the tiles in that area showed no obvious problems, but after removing the felt it was obvious that 1/2 of one sheet of OSB was wet, severely water damaged and would need to be replaced. Replacing this 1/2 sheet would not be a big deal, except for 2 complications:
1) Another triangular piece of OSB comes down over part of the sheet to prevent water from pooling in that corner of the roof and
2) A waterproof membrane behind the stucco wall in that area rolls out over the roof deck.
Having had some past experience with deck joins to Stucco walls, I am very reluctant to inadvertently cause any more damage than necessary to the Stucco or membrane behind the Stucco. Questions for the net at large are:
1) How should I cut or otherwise get the waterproof membrane out of the way, so that I can get at the OSB to replace it?
2) How do I replace the felt such that the problem does not re-occur or a new problem appear?
Comments appreciated
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You can't.
Moose out front should have told you.......
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Well, the real reason the leak started was mounting all the Christmas lights, but I was hoping people wouldn't notice that....
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Clark, Clark, Clark. You'll have to just go and remove the rotted OSB, replace with new, go _over_ all the old sticky stuff in that valley, plus, up that wall more than you'd like, with new sticky stuff. Then, you'll have to repair the stucco. Sorry to hear about the tar job...you'll have to clean the tile, or get new ones. Tom Work at your leisure!
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