You have a couple questions.

1) You can sail upwind by reading telltales on the most sensitive sail. The jib if you have one or the main, if that is all you have. Personally without a wind indicator, I usually over trim the sails.

2) The center of force. On most sloops the mast is raked aft. When you take the jib off, you normally have to rake the mast forward (not forward of straight up!). If the mast on the boat is far enough forward, this works fairly well, if not well....