You can't buy or rent a BiPAP without a prescription, legally. But as we have mentioned, any doc can write one. The difficulty is meeting reimbursement criteria if reimbursement is the goal. Otherwise, all you need is a script and a credit card.
There is no point in paying exorbitantly to rent a BiPAP when you can buy one for <1k.
Let's not turn this into a BiPAP thread, though -- we should try to stay on topic for future readers.
Anyway, if Tom's unstable in bed, a mask and a hose could be just more issues in the mix. So I would address the positioning first.
As for removing the sling, we did that by using the bed head/feet, just as in the wheelchair. After removing the sling from under the legs by lowering the feet, you reset the foot of the bed, lean him forward after dropping the head and remove the back of the sling.
I don't believe in leaving the sling under anyone in any chair or bed. Pressure injuries in ALS can be forever just as in SCI. And it's not necessary.