does Petr have negative value

yes

it really depends. he did have some signs of being okay this year but hes just been injured, they might be one or two gms that would pay something small

imo he has positive value still. he came back strong after his second injury, only to get injured once again. if he's fine after his last injury then he's positive value, otherwise no he has negative value
feel like a team low on goalies (ARZ, NJD come to mind) might trade something minimal for him

Mrazek is the Leafs starting goalie next season.

Could probably easily deal him for a player with similar term and cap hit.

Really depends on how the backup goalie market in UFA ends up