1. It’s a choice. You are allowed to say “Ya Muhammad Madad” I guess we mention “Ali” because of the oppression done him and how enemies didn’t want anyone to mention Ali. Help is indeed from Allah and Ahlul Bait are our main ways to Allah.
2. There is nothing wrong so long as they are sincere servants of Allah like Bibi Zaynab, Bibi Masumah, Hazrat Abbas etc. Remember help is from Allah and they are means to Allah
3. They may not be among the 14 infallibles but one cannot point any sin they committed. They were devoured servants of Allah.
4. Quran 4:64
5. I haven’t come across that but that doesn’t mean we cannot so long it’s not with the intention of worshiping an Imam but rather as means of approach to Allah