Let me first note that assisting at general happiness is an odd locution. Generally, when we assist at something, it's some action or event, not an emotional state. However, that's a semantic consideration. Both of your sentences are grammatical.
Secondly, the imperfect isn't generally a label attached to English tenses. We have a simple past (assisted) and a past progressive (was assisting). The former generally indicates a completed past event; the latter, a past event that occurred continuously over some period of time.
As you note, there are two imperfect aspects -- the continuous and the habitual. In English, the former uses the present participle (ending in -ing). Thus if you asked a doctor about his work last night, he might say
 I was assisting the chief surgeon.
and you would understand that this was a continuous effort, that is one that took some time.
The latter has several modes of expression. Thus if you asked a doctor about his medical career, he might say
[2a] When I was on staff, I would assist the chief surgeon.
[2b] I used to assist the chief surgeon.
[2c] I assisted at operations.
In each of these, you would understand that the doctor repeatedly assisted during surgery. Note that 2a relies on a modal use of would and that 2b and 2c pick up the habitual aspect from the semantics.
The situation for the present tense is different, especially for stative verbs. The simple present often represents ongoing repeated or habitual actions. Thus if I say
[3a] I go to the gym.
I mean that I have often exercised at the gym and will continue to do so. If I say
[3b] I am going to the gym.
I mean either that I'm enroute right now (the present) or that my departure is imminent (near future).