MC and ML separated because ML's mom confronted and scared MC. MC fled to his hometown and waited for ML to find him. ML went after MC but ML's memory was tampered with by Grandpa Lu. So they separated for many years cause well meaning family members on both sides were punks.

MC and his family can transform into creatures of the Chinese Zodiac. They have special physiques and powers. Therefore, Zaizai required expensive medicine to maintain his human form. MC works several jobs to provide for their needs. He is a wonderful daddy.

It seems reasonable because the grandfather is not human in the first place. He's like an immortal who has been alive for hundreds, maybe even thousands of years. MC and his family are mythical creatures! Now, I also don't understand why MC and his child had to suffer for seven years alone when he was so many awesome siblings, who definitely weren't poor. But maybe it has got to do with how they were raised or how MC chose to experience life when he went down the mountains. To us, those years of juggling multiple jobs would've been definitely difficult. But for MC, it's just part of going through life.

Again, the grandfather is not mortal. Seven years might not have been too long. For him, for the MC, for the MC's son. But yeah, I get the anger and unfairness because while it might not have been long for him, it was definitely long for the ML.