This kind of metonymy is very common in Latin.
For a simple example, vir mortuus is literally "a dead man" but can also mean "the death of a man".
This is somewhat similar to how summus mons can be "the highest mountain" and "the peak (= the highest part of a mountain)".
The point is that reading very literally can ...
The ea (= eā) modifies causa, using the very common adjective–preposition–object of preposition arrangement: 'for this reason.'
The forte is from the noun fors, 'chance' (not the adjective fortis, 'strong, brave'); so the ablative/adverbial form means 'by chance.'
Yes, this does indeed appear to be a partitive genitive.
Changing the word order as you suggest is legitimate if it helps you.
I think it is most useful in its original place where una and sine alia are next to each other.
I like analyzing things sequentially, so let us start with the whole sentence.
To clarify the role of the relative clause, I will add ...
Literally, '...they do nothing different than if someone... (makes the other conclusion about a circle)'
Or, to translate a bit more loosely, '...what they do is no different than what some other person does if he/she... (makes the other conclusion)'
Or '...they act no differently than if some other person... (makes the other conclusion)'
The adjective ...
1 - I don't think Latin makes much of a distinction between substantia est corporea and est substantia corporea. In either case, I think you'll need to rely on context to determine which English translation to use.
2 - Eatenus is the correlative to quatenus. Both translations use "insofar as" to translate quatenus. It seems to me that there is nothing wrong ...