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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.