Yet another answer

b1[x_] := Module[{f},
  f[{x, R_}] = True;
  f[{L_, R_}] := f[R];
  f]

Block[{$RecursionLimit = 1*^6, $IterationLimit = 1*^6},
 {
  MemberQ[ll, {86, 99}, Infinity] // timeAvg,
  t7[{86, 99}][ll] // timeAvg,
  b1[{86, 99}][ll] // timeAvg
  }
 ]

{0.0196000, 0.00751200, 0.00321600}

Your move, Mr. Wizard! :)

Yet another answer

b1[x_] := Module[{f, res},
   f[{x, R_}] = True;
   f[{L_, R_}] := f[R];
   f[#] /. f[{}] -> False
   ] &

Block[{$RecursionLimit = 1*^6, $IterationLimit = 1*^6},
 {
  MemberQ[ll, {86, 99}, Infinity] // timeAvg,
  t7[{86, 99}][ll] // timeAvg,
  b1[{86, 99}][ll] // timeAvg
  }
 ]

{0.0196000, 0.00751200, 0.00321600}

Your move, Mr. Wizard! :)