1- correct.
2- The broker has a certain amount of time to report all fills/trades to the exchange. He will remain short until he decides to cover the 100,000. In all likelyhood he will advertise that he's a buyer with an indication of size on an institutional network and has his salesforce make selective calls to known holders to see if he can entice a seller and match up a natural seller to his "new" short. In a stock trading millions of shares per day he'll have no trouble getting flat or finding a cross trade.

Keep in mind he's getting several cents per share as commission not like the $7 at a Scottrade regardless of size AND he gets a few cents extra to commit capital by shorting shares to the institution.

Yes I ran the trading desk for a large buy side institution until my retirement. But , it's been several years so commission rates and instutional trade and advertising order systems have undoubtedly changed somewhat.

Another little known fact is many buy side traders are graded by beating the VWAP (volume weighted average price for the day). So if they have been buying a stock all day and it's moving up they can gun it at the end of the day with the balance of their order knowing their average purchase price will be lower than the closing price. Same on the sell side. That's why you can get directional follow through in the last several minutes of trading. I refused to grade my traders that way. Sorry if it's too much info.