Learn Italian: Mio nonno diceva che si è faticato molto in questo mestiere. - My grandfather said that people toiled a lot in this trade.

The verb faticare (to toil or work hard) normally takes avere in compound tenses (for example, lui ha faticato). However, when you use the impersonal si (meaning "one" or "people"), the auxiliary verb always shifts to essere. Therefore, it must be si è faticato.

Since the auxiliary shifted to essere, why doesn't the past participle become plural (si è faticati)?

With the impersonal si, the past participle's ending depends on the verb's normal auxiliary. Because faticare normally takes avere, its past participle remains in the default masculine singular form (faticato). It only changes to the plural masculine (-i) for verbs that normally take essere, such as andare (si è andati).

Why is diceva in the imperfect tense rather than the passato prossimo (ha detto)?

Using the imperfect diceva implies that this was something the grandfather said repeatedly over time, describing a habit or a recurring action in the past ("he used to say"). If the sentence used ha detto, it would mean he said it just once on a specific, finished occasion.

Why is there no article before ? Shouldn't it be ?