(2). That is not what I said. To sell at P+dP, you need to buy up all the shares at P+dP as well. That's because of price-time priority, which almost all the major markets have (with some exceptions, most notably NYSE -- and even then, their book is mostly price-time priority). This is the idea that a person who placed their order first, has priority over those who placed their orders after. A strategy that makes the trade that you are proposing must then also take out all of P+dP and sell against the incoming order. In a usual case, you pay commission when trading into that position, and (maybe) earn commission when you sell it back (as the liquidity provider). In no market is the commission earned greater than the commission paid. The shares you buy-and-sell at P+dP are straight up loss, and that's before figuring out if X (the incoming shares that you are supposedly front-running) is greater than the #shares at P+dP anyway.
"The risk he takes only depends on the size of N" -- that is true -- that is always true, if you are making an unhedged trade.