The sputtering / spitting / splattering doesn't always mean its butane or c02. It could be and most likely is moisture.
some strains won't shatter, so even if there were low temps you might still not get shatter.
i agree double boiler would work easier but depending on your ovens settings there might be a proofing bread option, and that might satisfy not going above 115f.
Maybe try putting the whole PP with the shatter in the oven at 110f-115f then scrape it off while warm. your concentrate most likely was at these temps when it was purged anyhow so you shouldn't lose too many terpenes.