Ideas for leaking metal roof- lap joint

Long story short- roof is 65 feet across, 45 feet down the slope, very shallow slope (perhaps too shallow), the lap joint that runs horizontally across was done wrong years ago.
Previous owner's contractor ran the sealing tape above the screws instead of below, so the water wicks up the joint and down the screwholes. As a temp fix I've tried sealing the joint, but the joint moves quite a bit with temp changes, so come spring I have to do a more permanent fix.
I'd like to remove a section of the panels across the middle of the roof (going horizontally) so I can fix some of the rotted plywood that's under the joint. Then I'd add a new panel (under the upper panel, over the lower) so it'd be 3 panels down the run of the roof instead of 2. 65 feet worth of pulling screws from above the joint, prying the metal up, sliding the panel underneath, putting on the sealing tape, and screwing back down. Seems pretty feasible except for the cut.
This would mean making 2 cuts (each 65 feet long) through the existing roof panels, but how to make a cut like that that won't leave a jagged edge, or cut through the plywood and into the trusses? I don't think there's a 'neat' way to do it, pretty much would have to use a circular saw with a fine-tooth carbide blade. Would go over it once with the blade set 'deep' but just cutting the high parts (peaks) ___/ \\___, then again with the blade set shallow to get the low parts.
I can punt and not replace the plywood (the weak section is quite narrow), but I'm not sure I'll be able to get a good seal if I just slide a new section of roofing under the uphill side, and over the downhill side. Seems like having the roof 3 layers thick at the existing lap joint might be problematic. That narrow horizontal strip of roof won't hold a screw, either.
Yeah, I know, just have the 45-foot by 65-foot roof redone. Not in the budget. Its an easy one though, only 1 penetration (masonry chimney), and its the highest part of a shed-style roof. No peak, no wall interface.
Maybe I need to pull the metal roof and do roll roofing- maybe the pitch is too low even for a properly installed metal roof? Its about a 6 foot rise (or so) in 45 feet. 6/45, that's a 1 in 7.5.
Any ideas/strategies I'm overlooking?
Dave
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On Dec 28, 8:24pm, snipped-for-privacy@yahoo.com wrote:

Just had an idea to simplify the cutting- make a jig for the saw from some lumber that will guide the saw straight, and hold it a set height above the panels so it can't overpenetrate, but can cut it all in one shot.
Just thought of another downside- won't be able to properly re- tarpaper the involved areas.
Dave
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On Dec 28, 8:30�pm, snipped-for-privacy@yahoo.com wrote:

how about roll rubber roofing glued well over the bad area, to stop all future leaks. wouldnt fix damaged wood but perhaps its not critical
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I fear it'd be hard to get it to seal over all the little peaks, and this joint moves a lot- I mean a lot. The metal panels are over 20 feet long, so there's a lot of expansion/contraction.
If I built up a little 'bump' or upwards loop in roll roofing to allow for that, it'd build a pretty good dam, and because of the shallow slope it'd back up a good distance, and start putting screws under water with predictable results.
Dave
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