I enjoy your educational posts. I'm a native English speaker. In my experience, I have never heard anyone use "cummed". It's always "came" or e.g. "did you cum". It's common to use "I'm coming" in innocent contexts, though those of us with dirty minds may be conditioned to think of it sexually even then. But it's not something that will usually be made fun of unless you're with friends who make sex jokes commonly anyway. However, it may be more difficult for a non-native speaker to use it in an inconspicuous way. And yes, "I'm coming" vs. "I'm cumming" is fairly interchangeable. My personal interpretation is that the latter is more vulgar, and to me, it more strongly implies ejaculation.
Bobogoobo
I enjoy your educational posts. I'm a native English speaker. In my experience, I have never heard anyone use "cummed". It's always "came" or e.g. "did you cum". It's common to use "I'm coming" in innocent contexts, though those of us with dirty minds may be conditioned to think of it sexually even then. But it's not something that will usually be made fun of unless you're with friends who make sex jokes commonly anyway. However, it may be more difficult for a non-native speaker to use it in an inconspicuous way. And yes, "I'm coming" vs. "I'm cumming" is fairly interchangeable. My personal interpretation is that the latter is more vulgar, and to me, it more strongly implies ejaculation.