I am sorry for the loss. Like other said you cannot leave without AP or else the application is considered abandoned and you will not be able.
I read somewhere that you can get an Emergency AP for 30 days at your field office for situations like this. Try to search on VJ, I could not find the post again sorry and if you can try to go to your field office and see what they can do for you it would be best.
You need the AP in hand before traveling abroad. The person at uscis that told you can leave after you did your bio should be fired IMO. This person does not know if you will be requested to come back to do the biometrics again. It did happen to me at my AOS stage. If your grandparent funeral is more important to you then go and take the risk. Likely they won't let you in without the AP and you will have to start all over again but with a spouse visa. I know it sucks to have to choose between staying in US while your AP is processed or being in your home country for your grandparent funeral. Even if you have your AP expedited, probably it would not be in time for the funeral. The process take so long that lot of people miss important events like weddings or funerals in their home country. Hope you can work it out.
