For -ger verbs (e.g. nager, changer, corriger) the e is kept in the nous form to keep the g soft, but is the -eons ending pronounced exactly the same as the -ons ending ? Are there any exceptions?
Phonetically there is no difference between nous partageons et nous parlons regarding the -eons and -ons endings. It's like you said. Keep the g soft.
Note that this e is not introduced in all the tenses and persons.
See, e.g. the conjugation of partager.
Nous partageons mais nous partagions (imperfect tense).
je partageai tu partageas il partagea nous partageâmes vous partageâtes