I used to have a real helicopter strop permanently rigged to a block and tackle when sailing with kids on board. I bought it for next to nothing in a military surplus junk shop in Devon.
Lifting a MOB out vertically is not recommended practice (ST series).
Blood gravtitates when vertical starving the vital organs. The Tribuckle sling or the storm jib and parbuckling seem to be favourite.
It depends on how long the person has been in the water whether a vertical lift is viable.
This dates back to the 1979 Fastnet when crew were dying when being lifted into the helo due to extra pressure on heart following hydrostatic pressure.
Tribuckle, storm jib etc take time to rig, it's difficult to get casualty into the sail/device. Useful perhaps for unconscious or dead casualties.
Suggest using a sling in conjunction with a safety lanyard behind the knees to bring MOB up in seated position (if they've been in the water for some time).
Any lifting and hoisting supplier sells polyester slings like these We added some padding to make ours a little more comfortable.
Unfortunately the Tribuckle is no longer in production. A worthwhile alternative, at a much more affordable price, is the perforated plastic netting-like stuff that road works are fenced off with. Lets water out easily, attaches easily to toerails and halyards and is plenty strong enough for the job.
When we tried a similar exercise for ourselves it highlighted a number of the points that you found. Our original intention was to use the mainsheet in its normal position, taking the extra long free end to a winch. We found it impossible to arrange a good lead for the rope without riding turns on the winch. Following some rearrangement we inverted the mainsheet, making the hoist far easier without the need for a winch.