Hello! After reading through many topics about LORs, I was hoping that someone could help/offer me your opinions.

I am 4 years out of university and I have one letter in the LSAC system from a professor. I had planned on getting a second letter from another professor that I had worked closely with (2 classes + 2 independent studies), however, due to some health issues and oversight from him... I don't feel comfortable asking him for the letter for a third time. (I had asked him twice, followed up each time, but his response to any request for the LOR is to ignore... if I email him about anything else, life updates, etc. he's all ears.)

Therefore, is it okay just to have one letter from a professor whose class was negligible to my credit hours?

Also, is there a specific type of class/professor I should be looking to get a letter of rec from? (I went to an engineering school, so I figure I should be looking towards the humanities professors.)

Side note, after working at a fortune 500 company for a bit, I moved back to my home state to work for the 'family business'. Is it in poor form for me to get a letter of rec from a colleague (unrelated to me in any form; we have a great working relationship/have worked on many projects together)?

Apologies in advance for how awkwardly worded this is... but thank you for any insight!