The transferred assets have to be allocated no less rapidly than ratably over 7 years. So you have to do at least 1/7th in the first year, then 1/6th of what remains in the second year, and so on. 415 limits permitting.
You can use a non-safe harbor method to allocate, as long as it is permitted by the plan document and satisfies all applicable testing.
I don't know that the IRS has ever actually come out and said that the 20% excise tax applies on any amounts that remain unallocated after 7 years. However, to be on the safe side, I would suggest not leaving any unallocated assets if at all possible, meaning get everyone to their 415 limit rather than leave unallocated assets.
No. It's not a forfeiture account. However if the plan generally pays certain expenses, I don't see any reason why those expenses couldn't be charged against the QRP account the same as any other account.