This is more a series of comments than an answer.
First, don't get distracted too much by Durant's small list. For example: yes, "essence" appears in Id1 (cf. SEP on referring to the Ethics). In Latin, he writes:
Per causam sui intelligo id cujus essentia involvit existentiam sive id cujus natura non potest concipi nisi existens.
By self-caused, I understand that whose essence involves existence or that whose nature cannot be conceived except as existing (My translation)
It's hard to imagine that there's an extant English translation that renders "essentia" as anything other than "essence."
Id4 is similar:
Per attributum intelligo id quod intellectus de substantia percipit tanquam ejusdem essentiam constituens.
By attribute I understand that which the intellect perceives of substance as constituting its essence.
When Durant says "he uses the term substance where we should write reality or essence", he isn't claiming that the words "essence" or "reality" don't exist for Spinoza—both essentia and realitas are technical terms in Scholastic Latin—what he means is that where Spinoza has written 'substance' (substantia) he uses it in a way that is closer to how we use "reality" or "essence" rather than how we use "substance". Thus, for example, when he writes (in Ip14) that "besides God no substance can be granted or conceived" (Elwes translation), it's fairly obvious that "substance" here means something other than what we normally take it too. Similarly, when Spinoza defines "substance" in 1d3:
"By substance, I mean that which is in itself, and is conceived through itself: in other words, that of which a conception can be formed independently of any other conception." (Elwes translation)
This sounds very little like how we tend to speak of "substance". Among other things, why does "conception" enter into this definition of "substance"? Of course, whether it in any obvious sense sounds like "reality" or even "essence" can only be seen by following how Spinoza uses "substance" within the Ethics. And here we come to what is actually Durant's point, which he states pretty clearly in the paragraph after the one you quoted:
Spinoza is not to be read, he is to be studied;... in these brief two hundred pages a man has written down his lifetime's thought with stoic sculptury of everything superfluous.... You will not understand any important section thoroughly till you have read and pondered the whole (p. 186–187)
The point isn't that Spinoza's language is outmoded or that his Scholastic terminology distorts what he's trying to say. Its that Spinoza's writing-style in the Ethics makes virtually ever term he uses imbued with odd meanings and it would sound just as odd had he written it in Dutch or even English.
