Ocheiz b'ama

The gemara on nidda 43a asks how could the mishna suggest that a man should be ocheiz b'ama instead of just ruining the teruma if being ocheiz b'ama can bring a mabul to the world. Tosafos Harash can't understand the gemara's question - being metamei truma is an isur d'oraysa while being ochez b'ama is at best an isur d'rabbonon. There's a little aleph in the Tosafos Harash which points to the Cheishek Shlomo at the bottom where two answers are suggested. The first is that even though it's "just" an isure d'rabbonon it still might outweigh the d'oraysa because it's meivi mabul l'olam. The second answer proposed is that shev v'al taaseh would be not to be ocheiz b'ama.

I have trouble with both answers. Meivi mabul l'olam on a d'rabbonon, I would think still can't outweigh a d'oraysa. Also, is it really shev v'al taaseh? There are three choices here:
1. spit out the teruma
2. finish eating it and eat tamei truma
3. be ocheiz b'ama
All 3 of those involve an action. Once an action is required shouldn't we choose option 3 since that's only an issure d'rabonnon. I guess that the cheishek Shlomo is assuming that just continuing to eat is a shev v'al taase but it does seem to involve an action of continuing to eat. I think that Tosafos Harosh' question is still very good. Of course, in the end the gemara has solutions which make it that you don't have to be ovair on any of the three so that's obviously ideal.