Clark and Chapman were like brothers. Chapman threatened to give up racing after his death and took little interest in it for months afterwards. It was Graham Hill who held the team together and I think when he won races and the championship Chapman became interested again.
Cevert's death certainly damaged the Tyrrell team. Stewart reckoned there was little between Cevert and himself in that last season and reckoned that at times Cevert could have beaten him. Losing Stewart was always going to be a big blow but the succession was in place. Cevert dying in what would have been JYS's 100th and last race was not only a tragedy but it meant the team had lost both drivers effectively on the same day. So from being the dominant team Tyrrell went backwards and never recovered. It is one of the great imponderables about racing to consider what may have happened had he lived.
Would Ferrari have enjoyed the success it had in the second half of the 70s or would it have struggled for much longer? Bear in mind that prior to Lauda's first championship in 1975 Ferrari hadn't won a driver's title since Surtees in 1964. It is one of the great myths of motor racing that Ferrari has been successful for most of its existence. In reality it was successful in the 50s, the second half of the 70s and when Schumacher and co started winning. Most of the rest of the time it wasn't that special.
I see that youtube has a clip of Cevert's death which must be one of the most awful racing accidents ever. No way I well ever watch that. They also have Tom Pryce's accident as well.