I think Widmore knew Des was on the Island. He knew about the Island, he had been to the Island, Des takes libby's boat (who had been on the Island), and des was on Widmore's solo race. Widmore was trying to maroon Des to keep him from Penny. he tried to pay him off, but it didn't work.
I'm sure Widmore knows how they all got off the Island too.
You brought up
one of many of my biggest questions. If Charles planed on putting Desmond on that island with that boat then how would he do that if he himself is still looking for the island?
Maybe this is a good topic to start a thread about. ya know?
Good point! If Widmore sent Des he would have to have known where the Island was in 2001. Perhaps the Island wanted Desmond for some weird twist of fate for Widmore. Maybe Widmore knew of a way for the Island to draw an individual in fairly easily, but not allow a freighter full of mercenaries onto the Island. As we have seen, it is not exactly easy for someone to just walk in and takeover the Island.
I am going to further speculate on my two groups of others theory that I have speculated on in another post because your thoughts are playing well into supporting my theory (which in a nutshell is that there are two groups of others, richard alpert others and widmore others, and they are vying for control of the island).
Now, we still don't know what happened after the purge but there is a good chance that the Richothers sent someone to turn the donkey wheel and move the island, which would have completely cut Widmore off. I don't believe that Widmore has the island's best interest at heart like Richard Alpert does, and that is why Widmore created the Dharma project with Alvar Hanso. The experiments they were conducting hurt the island, and once the purge happened the Richothers took further steps to protect themselves.
Following that line of speculation there is no way that Widmore could have sent Desmond there. I think there is a good chance that annual race of his was a front to see if any boats ever went mysteriously missing in the Pacific to give him a starting point on where to search for the island.
I think the Island drew Desmond to it, to act as a protector of sorts (or who really knows why that Island does anything, it does seem to have a strong survival sense though).
As for how Widmore found the island to send a freighter full of mercenaries, I am guessing the same way that Penny did. Through the Electromagnetic Pulse that the Swan gave when it imploded.
It just all seems to come together for me. The people who know the most about the island want it the worst, and it would only naturally make sense that it would be people who had inhabited the island at some point and were fully aware of its powers.