TCM...if the casualty has been in the sea for a while and/or they are cold, they may well be too exhausted to climb up a swim ladder; further, in a heavy sea that could be unsafe.
Love your idea of 80m floating line...! Excellent!
No. Centre sheets can work depending on the how the traveller block is attached i.e. removable or integral with the car. There could be advantages around centre sheets regarding lifting point position to the beam, topping lift use, boom control.
A parbuckle such as the old Tribuckle works very well. If pre-adjusted to the stanchion spacings at your chosen rescue position, it is quick and easy to set up and use.