so the limits work in the following way:
buy with a limit of 17.50 means buy when the price drops to 17.50 or lower (if it is currently lower it will happen immediately)
sell with a limit of 17.50 means sell when the price reaches 17.50 or higher
short with a limit of 17.50 means short when the price reaches 17.50 or higher
cover with a limit of 17.50 means cover when the price drops to 17.50 or lower
Each transaction is atomic and is based on what you have in your portfolio at the time of placing the order (e.g. you can't set 2 limit orders one to buy at 17 and then another to sell at 20).
For a buy or short order, you need to have the available cash at the time of placing the order - you can then use the cash for other things, but the order will fail if the cash is not made available again. For example, if you want to cover and then buy when a stock falls to 15, you can set those orders up as long as you have enough cash to buy the quantity of stock you want to. But you can then use the cash for something else - however there is no guarantee that the cover order will be done before the buy order, so the buy order may fail if the cash isn't there.
I use limit orders a lot to change my port position automatically after the star bond adjusts (if the stock is halted the limit condition gets evaluated when the stock is made active again) and have never had any problems with cover/buy or sell/short combinations failing, but there have always been other straight sells and covers at the same time.
Hope that helps.