First, I would HIGHLY recommend that you read through the archives in the Wholesaling forum.
As for your questions,
1) Remember that price overcomes all objections.
If you are selling the property where the buyer can profit $15,000 would they be willing to pay you $5,000? Would you pay $5,000 to make $15,000? I know I would, and any investor in their right mind would too. Now that doesn't mean they won't try to negotiate with you, but they'd be a fool to walk away from a REAL deal just because you are making a profit on it. Keep in mind you will meet some fools.
Remember that the buyer wouldn't have saved thousands of dollars, they would have never known about the deal.
Now part of this whole issue is why I prefer to double close. On a double closing the buyer and seller don't know what you're selling it for or normally that you are even reselling it.
2) On an assignment, you just pick up a check. On a double closing, yes, you would go to the closing.
3) Yes the deal can work, but it has more risk of falling apart. In a situation like this, I make sure that my buyer understands that I fully expect him to close either way. I also have at least $1,000 earnest money from the buyer, so if the deal falls through I keep it.
4) On an assignment, normally you would want to get some or all of the assignment upfront. The balance, if any. would be paid at closing. On a double closing, other than your earnest money you would get paid at closing.
5) On an assignment, normally I will prep the seller that someone else might be buying the property, but I always reaffirm that the seller has nothing to worry about whether my buyer buys it or not because I will either way. Normally, the seller is not going to care. They just want to get their money.
The different parties to the transaction normally sign at different times, so it really is not a big issue that they will see someone else at the closing. Now if its an assignment they may notice a different name on the HUD 1 closing statement and the assignment fee will be on there also. So that may come up as an issue. Some title agents will prepare a one sided HUD 1 that doesn't show the buyers side to the seller and the sellers side to the buyer. Either way the contract is still legal and binding, and the seller has a legal requirement to sell the property under that contract whether you assigned it or not.
Again a double closing insulates this information from the buyer and the seller.