1 -yes as far as i have currently tested only parasites cause it
2-no purify will heal the damage but not clear the unmarked status (i also tried the blast cleanser to no effect)
3- yes, after repeated testing it was the sweep attack that caused it but only if it downed its target. (the parasite downed Luca with dark orb a few times and Flavius with silence once, no effect on ether when they were revived and healed to check for dot damage) Edit: the debuff resets if the affected unit is downed by the sweep skill from the parasite. If downed by any other source the debuff does not refresh, it continues at which ever point it was at times and D.o.t wise once the unit is revived.
4- i have shuffled my party a few times testing but the 1st time i noticed was with Rei, Gillino, Ace, and Luca. after reshuffling my party to Heckrona, Ace, Luca, and Flavius , Ace went down to the sweep attack and Luca to dark orb but only Ace was effected with the debuff
5- no. when Leon goes down to the parasite ('s sweep attack at least) i get an instant game over 'taken over by the parasite' ending
6- yes i did have the new shard on Gillino but Rei was the 1st unit effected , and i did not notice anything wrong with Gillino till he went down to the parasite and was revived (unlike HP loss though items with a natural drain, the debuff ticks the screen red like when taking poison damage)
no worries, i'm doing the easy part of bug hunting. exploring, enjoying, and messing with in game things in multiple ways. you got the challenging part of understanding what when wrong why, and if and how to fix it.