1. are there any disadvantages of lending out the shares? eg if want to sell stocks lent out?
No problem to sell stocks lent out, one issue is as a long you might be better off keeping the shorts out of borrowing opportunities, but you are unlikely to change much as an individual on a stock like AAPL
2. what % return can we expect?
If you expect IB to find a borrower for you, on AAPL, ime, 0, I´ve had shares for a while and can´t remember they ever got lent out. Borrowing fee probably no more than 0.1% per year (fwiw it shows 0.35 on an AAPl ticket, but never noticed fee rates below 0.25 on the tickets, yet in account statement, the fee is around 0.04% on many large cap liquid stocks), of which IB keeps half.
3. do other option brokers do this?
Dunno