iubeo may take the infinitive or the subjunctive clause, but the infinitive would probably be better to avoid your stacked ut-clauses. i.e. eum iussit ut veniret → iussit eum venire.
ut colles Sabinos equitarent means “so that they might horse-back ride the hills”.
Setting out at first light, they arrived there as the sun was setting.
prima luce profecti, ibi advenerunt sole occase.
(should this be an ablative absolute or something else?)
ibi means “there; in that place”. With verbs of motion you usually need a word which means “there (thither); to that place”. Some words for “to there” are illo, illuc, and illoc.
You could use an ablative absolute for “as the sun was setting”. occase could only be the vocative of occasus, -a, -um though, so I don’t think it will be in that case no matter how you word it.
The next day Maecenas said that he must return to Rome.
postero die Maecenas dixit se ad Romam redire debere.
Roma is the name of a city, so you need not use a preposition to show motion.
Quintus was delighted by everything that he saw.
Quintus delectatus est ab omnibus quibus spectavit.
ab introduces personal agents; if the agent is a thing you may just use the simple ablative.
In the relative clause “that he saw”: “he” is subject, “saw” is verb", and “that” is direct object. quibus should be the direct object, so it needs to be in the proper case for a direct object.
He asked her why Augustus had summoned him, but she said she did not know.
eam rogavit cur Augustus se arcessivisset, sed illa dixit se non scire.
non scire = nescire. You could also express the negativity with negare instead of dicere.
I rejoice that you so trust me. But I am not worthy of so great an honour, and I am afraid that if I were to do this, I would not have enough leisure for composing poems.
gaudeo te mihi adeo confidere. sed indignum sum tanto honore, et timebam, si hoc fecerim, non satis otii ad carmina componenda habeam.
timebam is not the right tense for “I am afraid”. Usually the conditional construction “if ___ were ____, (then) ____ would ____” in Latin has both verbs in the imperfect subjunctive. I won’t comment on the fear clause and condition overlapping, since I’m not sure myself how best to write it.
non satis = parum (+ gen.)
He would call the farm manager and arrange for the fields to be ploughed; he would send out boys to guard the sheep.
vilicum vocabat et curabat agros arandos; pueri emittebat ad oves custodiendi.
“he would call… and arrange” and “he would send” are probably meant to be translated as subjunctives. pueri is the direct object of emittere, so it is in the wrong case. oves is accusative so custodiendi needs to be made to agree.
You could scarcely say that Quintus was an idle man, but he was content with his life and was always sad when he had to return to Rome.
vix dicere potuistis Quintum ignavum virum esse, sed vita contentus est et semper tristis erat cum Romam redire deberat.
contentus, -a, -um has diverged in sense from the verb contineo, continere, continui, contentus to the point where it is really more adjective than participle, so est does not put this sentence into the past as it would with a passive participle.
cum + indicative shows a specific time. “when he had to return to Rome” is not a specific time, since he obviously was going back on forth between Rome and the country at multiple times, so you must use subjunctive. deberat is not a word (debeo, debere, debui, debitum).