either

  1. There’s nothing wrong with that either method.
    1a. There’s nothing wrong with that either methods.
  2. There’s nothing wrong with either of these methods.
  3. It seems unlikely that either is true.
    3a. It seems unlikely that either are true.
    Please correct the above.

There’s nothing wrong with either method.
There’s nothing wrong with either of these/those methods.
It seems unlikely that either (one) is true.

How about:
There’s nothing wrong with that method either.

?
Thanks!

That’s correct. It has a different meaning to my sentences above, but it might be what Allifathima was looking for.