If you're using Rails, the ActiveSupport::Multibyte::Chars library (#mb_chars on any string) passes all these tests except the upcase of baffle:

  irb(main):001:0> RUBY_VERSION
  => "1.9.3"
  irb(main):002:0> Rails.version
  => "3.2.14"
  irb(main):003:0> # example 1
  irb(main):004:0* example1 = "noe\u0308l".mb_chars
  => noël
  irb(main):005:0> example1.reverse
  => lëon
  irb(main):006:0> example1.compose.slice(0,3)
  => noë
  irb(main):007:0> example1.g_length #grapheme_length
  => 4
  irb(main):008:0> example1.compose.length
  => 4
  irb(main):009:0> # example 2
  irb(main):010:0* example2 = "😸😾".mb_chars
  => 😸😾
  irb(main):011:0> example2.length
  => 2
  irb(main):012:0> example2.slice(1,1)
  => 😾
  irb(main):013:0> example2.reverse
  => 😾😸
  irb(main):014:0> # example 3
  irb(main):015:0* example3 = "baffle".mb_chars
  => baffle
  irb(main):016:0> example3.upcase
  => BAfflE
  irb(main):017:0> # example 4
  irb(main):018:0* example4 = "noël".mb_chars
  => noël
  irb(main):019:0> example4 == example1
  => false
  irb(main):020:0> example4 == example1.compose
  => true

    "noe\u0308l".size # => 5

ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin13.0.0]

Ah, oops. I read the post too quickly, now I see the problem.

You forgot the "l":

    "noe\u{0308}" # => "noë"