Questions on chimney removal and shingle roof patching

I'm removing my unreinforced brick chimney this weekend because it's a seismic hazard. I'll be left with a 17" by 26" hole in my 8:12 roof. There are rafters on either side of the chimney, so I'll cut back the decking to the center line of each rafter to install the decking patch. I believe I understand the basic roof repair process, but I do have a few questions:
1) The top edge of the chimney is at a ridge line, so I can nail the decking patch to the ridge board. If there is no blocking at the bottom edge, is it necessary to use clips or install blocking? Should I leave a gap between the new and old decking?
2) 15lb roofing felt seems to only come in 432 square foot rolls, while I need about 5 square feet. Is there an alternative underlayment that comes in smaller quantities? I'm envisioning some sort of self-adhesive rubber square.
3) The weather forecast for the weekend is "partly cloudy, high 53". Next week rain is forecast. Should I use a heat source to activate the sealing strip on the shingles? I'm using a "two-piece laminated fiberglass-based shingle". Is a plumber's torch an appropriate heat source? If so, how close can I bring the flame to the shingle?
Thanks, Wayne

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