Could you check this over

Ok, but ei generally becomes ē in Celtic, so does a former eh₁i also become ē or does it remain separate?

Also, the shortening of long diphthongs is because of w:Osthoff's law.

Presumably eh₁i becomes ē just like ei. Loss of laryngeals between vowels occurred early in most (all?) daughters, followed (usually) by early vowel contraction.