The construction
X on the off chance Y
Indicates that some action (described by a complete clause X) is undertaken in hopes that it brings about (but probably will not) another action (described by complement clause Y).
Y (which is required) must be a complement clause introduced by that (that could be omitted in informal styles, but keep it in for a research paper). A model usage would be:
David took the 7:30 train home on the off chance that he would see Mira.
Furthermore, Y usually has a modal auxiliary as its finite verb (could, would, might, should).
To reword your sample sentence so that it fits the template for this expression,
The main disadvantage of these approaches is that they tend to be undertaken on the off chance that they could succeed, rather than with definite expectations about when they will bear fruit.
A commenter has correctly pointed out that Y could also be a gerundive verb phrase introduced by the preposition of (e.g., "of succeeding", "of finding a way out of the cave"), as in
David took the 7:30 train home on the off chance of seeing Mira.
A slight variant of the construction is a type of conditional construction:
(If) On the off chance Y, X.
Which indicates some action represented by X that should be taken in the (unlikely) event that Y occurs. Here, Y usually does not have as its finite verb a modal auxiliary. e.g.,
(If) On the off chance that you see butter at the store, buy two pounds.
The linguistic or grammatical principle that likely provokes your concern is termed coordination. Here's Wikipedia's article on the subject.
There often needs to be some kind of match between the elements that are coordinated in a sentence. But many of us poorly understand which kinds of matches and mismatches are grammatically felicitous, questionable, or generally unacceptable. In fact, it's a challenge for linguists to specify a parsimonious set of rules governing this matter.
Linguists often use the following as an example of elements which are mismatched in terms of sentence part categories, but permitted because they match in terms of function:
He's [a Republican] and [proud of it]!
A Republican is a noun phrase, while proud of it is an adjective phrase.
We might therefore wonder if setting them in coordination is permissible. It is, in this case, because although they do not match in terms of sentence part category, they do match in terms of function: They both complete (complement) the subject.
Your example does the same thing. The prepositional phrase and the adjective phrase both complement the subject, and they match in a way such that they can be acceptably set in coordination.
This doesn't mean that all complements can be cast as coordinates. As alluded to above, useful guidelines on this are hard to come by.
Whether or not your example is good or bad style is more subjective. If people who are good talkers were to debate it, surely they would want to know whether it was to be spoken or written, in which sort of genre, and targeted toward which kind of listener or reader. In and of itself, there is nothing bad about the style. Its meaning is clear. To me.
You have confused parts of speech with sentence constituents. A part of speech is a label attached to a lexeme or "word", like adjective, preposition, or noun. A sentence constituent is a label attached to a meaningful part of a sentence which can be characterized as playing a functional role in the sentence's structure, like subject, complement, or object.
There are situations in which elements in a set need to agree in some way, some situations in which it's better style if they do, and some situations which are generally seen as bad style, ungrammatical or just nonsensical if they are divergent.
Fine:
He is red, of another planet, and father to 249.
The items in this "enumeration" or set of elements would be considered bad style by most:
Duties included:
1. Running the front office
2. Wrote computer programs
3. Punctual
4. Professionalism.
Ungrammatical or nonsensical (?):
The car contained peanuts, in, contain, and go tell your mother.
Their ages were 17, 32, 56, C, and yellow.
NOTE: Thanks to @snailboat for redirecting my original answer onto firmer linguistic ground insofar as identifying the relevant topic. She deserves credit for what I got right, and no fault for the answer's travels into Errorsville, Papland or Drivelvania.
Best Answer
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).