Varför så allvarligt?
I wonder if it is possible to use seriös instead of allvarligt
They're mostly synonymous, although it would sound weird in some contexts. But note that it would be seriöst, not seriös, because of the det.
Yeah, I'm going to report that. I wrote that but it wasn't excepted.
It's accepted now, I wrote seriös and it was accepted with a typo (seriöst)
is it allvarligt because det is used? would it be allvarlig if it was den? or wouldn't you use den in this kind of sentences?
If you were talking explicitly about an en-word, you'd use allvarlig, correct.