I wanted to comment on a couple of things here. First, I don't think you need to feel any guilt about being a working mother. My own mother was a SAHM and I raised the whole death question with her about a dead bug I found in the shower stall at the public pool. Yup, she got to have this discussion in public. Guess how old I was? Yup, three. I think this is a very very normal issue for three year olds to raise. Our son also had a lot of questions at three. Each child will at some point start to understand and think about death, completely regardless of whether their mother works outside the home or not.
We didn't go into a lot of detail about burial when we got these questions from my son. Though he also didn't focus his questions on that topic. We did talk about heaven, which I believe in, but my DH does not. So, I led the discussions. I really think it is also OK for these to be somewhat difficult unresolved discussions. So, as hard as it is, I wouldn't approach this as "the cat's out of the bag and I need answers quick." Which of us can claim to have come to our own understanding of death quickly? Can any of us be sure that we are finished understanding death? But it kind of reminds me of what my DH says whenever I get sad about the kids growing up - he says, they are *arriving* more and more every day, as they get more verbal and more able to talk about complicated things. I guess my point here is that you are going to be talking about death with your children off and on for as long as you are both alive. This is the beginning of the topic, not the ending.
For us, it was harder when we confronted an actual death. Our much-loved kitty died when DS was 4.5. He marched right through whatever that sequence of emotions is: denial, anger, sadness, acceptance, etc. He got through them MUCH faster than I did, which I think is also pretty typical for children. The thing I found most satisfying about this process was just being with him while he had those emotions. I couldn't bring our kitty back, but I could hold him while he raged against the vet and while he sobbed about her being gone forever. We had a little ceremony for her and talked about what a nice kitty she was. He didn't watch while we buried her - actually that was too hard for me too so DH did it.
I guess my point here is, this IS hard and you don't need to fix that. I think it's a great idea to reassure your little guy that his parents will be around for a long, long time. If he wants to imagine heaven as everyone living in a big house together, why not? If he really really insists that he wants to know about burial and how it works, I would probably tell him. If he throws a tantrum when he finds out that "only" your soul goes to heaven, well, he's hardly the first or the last to have that reaction to that news. I'd keep trying to emphasize that he'll not have to worry about this for a long long time.