Scientific/Technical paper: enumeration/listing questions

The reason for using ‘;’ rather than ‘,’ is to avoid ambiguity if the item/phrase includes a comma.

Whether to use small Roman numerals or Arabic numerals is a matter of style, and whether you are using numerals elsewhere in your hierarchy. I think small Romans here are OK as they are somewhat less obtrusive (you are just using the x-height) than Arabic numerals.

Obviously, be consistent. You are creating a hierarchy of ideas and people can follow it more easily if the structure is consistent.

Repetition is sometimes unavoidable. In scientific writing, it is more important to be unambiguous than to be elegant.

Avoid ‘elegant variation’ — calling something by one name in one place (e.g. “The Prince”) and then something different elsewhere (“Charles”) and something else somewhere else (“our hero”). It confuses.

But...

I would definitely remove the period (full-stop) from (i.) etc. You already have parentheses to separate the ‘i’ from the rest of the sentence, so the stop is quite unnecessary. I’ve never encountered it in any scientific journal in my field.

And by the way...

If you are writing for a journal, do consult the Instruction to Authors and examples of recent papers. This will answer some of your questions about the style to adopt. Different journals have different house styles, and they won't change them for you (or even for me).