Yes, technically it's a result clause, although in this case the result is clearly intended so you could argue there's a nuance of purpose as well.
One may note that sometimes clauses introduced by
ita actually are negated with
ne rather than
ut non because the sense of intent predominates (e.g. Cicero "ita corrigas ne mihi noceat"), so at that point one could quibble as to whether they are primarily purpose or result, and I think the same would apply here. In this case the priority seems to me to be to understand the sentence correctly, regardless of what you call the clause.
2- If the army were pressed they would have (the result would be) a retreat.
I think you're confusing the apodosis (the "then" part) of a conditional with a result clause; in the sentence "If the army were pressed, (then) they would have a retreat," the clause "they would have a retreat" is just the main clause, rather than any sort of subordinate clause at all. A result clause is a specific type of clause introduced by
ut or
ut non, not just any part of a sentence that shows a result.
In this sentence, the clause under discussion is
ut ... expeditum ad suos reditum habeant. The
si clause is a further subordinate clause within that clause, but it's not going to be what determines the type of
ut clause we're looking at. That depends only on the relationship between the
ut clause and the main clause.
Aurigae ita currus conlocant,
ut,
si illi a multitudine hostium premantur,
expeditum ad suos receptum habeant.
Ita...ut... is a common formula for a result clause ("x does y
in such a way as to produce the result z"), which is why this is grammatically speaking a result clause, though it clearly has a sense of purpose/intent in this case and it may well be that Caesar intended it primarily as a purpose clause.