You get an offer specifying the grades needed.
You get in if you make that offer.
The university expects a certain number of places filled with people who make that offer, then put a certain number of places up as available for lower grades, but it won't go above a certain percent, so the number of places offered on a course in total depends on the number of people who achieved the original offer.
( so if they decide 10% can be lower, and 30 students make the higher grade in marine biology, and 50 students make the higher grade in soil biology for example, they might offer 3 places at lower grades for marine biology and 5 for soil biology - they will want a certain number of students, but the spread across the subjects isn't very important - you won't get 20 lower grade students accepted into marine biology to make it equal to soil biology for example)
The competition for the lower grade places can be quite fierce, even if you just missed the higher grade place by one mark, you would have to compete for the lower grade places, show that you have something the others don't, better work experience, or similar.
Spreading A levels out over two years devalues the grades, so likely to need higher grades to be considered equivalent.
Unless the univeristy course chosen is sport related, then being so involved in sport is likely to count against you. ( some sporting involvement shows team work, physical activity, being a rounded individual - competing at national level, particularly if not in a team sport, can show a level of self absorption and a distracting priority)
I have known international standard athletes get university places in unrelated fields, it does happen
In my experience, bereavements and illnesses in the family don't lower the offers either, unless the student has been a carer. I know a student who missed an A level exam because his mother was having a psychotic episode and it wasn't safe to leave her. He was accepted with a lower grade.
To summarise - you cannot predict whether or not you will get in with a lower grade.
Several of the circumstances you seem to be expecting to mitigate will actually not mitigate, and may count against your DC
He is disadvantaged by splitting up his A levels, as well as by not getting the grades needed.
he should retake them all.
The original grades can still go into his application. His personal statement can still talk bout his sport ( but play it down) his reference can still mention briefly bereavements etc, but again play it down.
In my opinion, that would give him the best chance of getting into his first choice university.
Or he could just go through clearing and take pot luck - that can work out very well sometimes