1. the return air ticket is not mandatory, the immigration officer of Hong Kong will choose visitors to check the return air ticket, if you want to stay in Hong Kong for 90 days (visa free period), you'd better prepare the return flight ticket because the immigration officer stamp the entry and stay duration on your passport when you enter Hong Kong.

2. you can exit Hong Kong when the 90 days visa free period will expire, you can go to Mainland China or Thailand, then re-enter for another 90 days stay, however, if you do it twice, the immigration will notice and write 3 capital letters on your passport, which means that the immigration officer know that you frequently exit and enter Hk just for new stay duration.

if your employer is willing to help you to apply for work visa, you can get it, but as I know, some employers are not willing to help their employee to get the work visa, they want to employ some aliens who have held the HK work visa.