It is so hard withthe teething and colds etc AND the18 month regression. We are there too & it sucks! But if you can hold firm and be consistent it will help.
If you can I really would stick with GW rather than WIwO if you cando it as this is more appropriate for an LO who hasn't slept fully independently before, it is just a bit more gentle for them, as the name might suggest.
Also, if you've always worked a more 8-8 type schedule, I think a nap at 12 is possibly what is leading to some EWs. Lots of Los on a 7-7 pattern will nap at 12/12.30 at this age so if you want an 8ish BT and a later WU you may need to shift the nap slowly towards 1/1.30. What do you think?