sh_ and ch_ compose the same words

[nablator]

1. 
I assume that sh_ and ch_ words are the same, because they behave the same and have same contacts

Sh and ch are generally very You are not allowed to view links. Register or Login to view.. By "contact" do you mean the next glyph?

2.
If sh... words are always compacter than ch..words in the entire text,

If by compacter you mean shorter, by average EVA length of word type or word token, yes.

3.
could this strongly signal that sh...words are compacter because the diacritic mark above signals an abbreviation?

The difference in average length (less than 0.3 by my count) would only be evidence of something always missing in sh... words if both sets of words had a reason to have the same average length. In the case of an abbreviation, sh... and ch... words are not the same so there is nothing that suggests that the two sets of words should have the same average length. Even if such a reason existed, it could be argued that an abbreviation would offset the average length by at least one.