a is '\*bc'
b is '\'
c is '*'
${a##?*} removes everything:    || - matches one char, then all
${a##?"*"} removes \*:          |bc| - matches one char, then *
${a##\*} removes nothing:       |\*bc| - first char is not *
${a##\\*} removes everything:   || - matches \, then all
${a##\\\*} removes \*:          |bc| - matches \, then *
${a##?$c} removes everything:   || - matches one char, then all
${a##?"$c"} removes \*:         |bc| - matches one char, then *
${a##\\$c} removes everything:  || - matches \, then all
${a##\\\$c} removes nothing:    |\*bc| - matches \, but then second char is not $
${a##\\"$c"} removes \*:        |bc| - matches \, then *
${a##$b} removes \:             |*bc| - matches \
${a##"$b"} removes \:           |*bc| - matches \

Single quote tests:
${a##?'*'} removes \*:          |bc| - matches one char, then *
${a##'\'*} removes everything:  || - matches \, then all
${a##'\'\*} removes \*:         |bc| - matches \, then *
${a##'\*'} removes \*:          |bc| - matches \, then *
${a##'\'$c} removes everything: || - matches \, then all
${a##'\'\$c} removes nothing:   |\*bc| - matches \, but then second char is not $
${a##'\'"$c"} removes \*:       |bc| - matches \, then *

${a##"$b"?} removes \*:         |bc| - matches \, then one char
${a##"$b"*} removes everything: || - matches \, then all
${a##"$b""?"} removes nothing:  |\*bc| - second char is not ?
${a##"$b""*"} removes \*:       |bc| - matches \, then *
${a##"$b"\*} removes \*:        |bc| - matches \, then *
${a##"$b"$c} removes everything:|| - matches \, then all
${a##"$b""$c"} removes \*:      |bc| - matches \, then *
${a##"$b?"} removes nothing:    |\*bc| - second char is not ?
${a##"$b*"} removes \*:         |bc| - matches \, then *
${a##"$b$c"} removes \*:        |bc| - matches \, then *