Calculate late penalty and remaining grade.
Late penalties are normally deducted in percentage points of the maximum mark per day, not as a percentage of your score — a five percent per day penalty on an 82 becomes 67 after three days, not 70. Most institutions also set a floor, below which lateness cannot push a passing piece, and that floor only applies if your original mark was above it. Both rules are modelled here, and the deduction is capped so a very late submission lands at the floor or zero rather than going negative.
Deduction
Points deducted = days late x penalty per day, in percentage points of the maximum
Floor
Final mark = max(original - deduction, penalty floor), never above the original mark
Almost always, yes. Where a floor applies, the difference between a heavily penalised submission and a zero is usually the difference between passing and retaking the module.
Policies differ — some count calendar days, others working days only. Check the module handbook, because on a five percent daily penalty a weekend is ten percentage points.
Through a formal extenuating circumstances process, usually with evidence and usually before or shortly after the deadline. Informal requests to the marker rarely have standing.