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.'
The pronoun id refers to the event, so you can translate Deo id volente as "when/if/because God wants so/it".
The form id is neuter and thus cannot refer to lapis, and it is accusative (instead of the other morphological option, nominative) because it is the object of velle.
It is not unusual to have other words, especially an object, within an ...
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 ...