Low Sloped Asphalt Shingle Roof Remediation
A) No. Nothing short of an unbroken membrane will resolve the issue. Even if you were to tar each row of shingles, you'd likely miss a few spots and have long-term damage. The right solution is a rolled roof or rubber membrane, possibly underneath shingles if aesthetics are a concern.

B) See A. 

C) The question is too broad for the SE Q&amp;A format. Ask for estimates from your local roofers, or price a flat roof solution at your lumber supplier.

D) Sure. If you carefully pop up each shingle nail, you could put them on Craigslist for sheds, hunting shacks, or whatever. Many folks would be glad to have a decent roof for half price (or less). This would take some time, obviously.