League-of-legends – Smartcast and range indicators for certain skills

There is a hotkey H which is the equivalent of toggling "Stop", which stops your current action. I'm not sure whether this will do what you need in stopping the auto-attack, but it's worth a try. I don't see any other options that could remotely do what you desire, other than getting stunned.

Continually pacing back and forth within range is a preferable course of action once you learn how to do it without getting exhausted. This actually has a few benefits besides just preventing you from attacking - it makes you a harder target, and it also makes it less detectable to be seen creeping closer and closer towards your opponent to make a pot shot. Alternatively, it makes your opponent uneasy since you can easily be in range to take said shot, which makes them less inclined to stay close.

The biggest question when leveling as Fiddlesticks is whether or not you are jungling. A jungling fiddlesticks needs a maxed Drain to be keep his health up, and should probably level Terrify to give him a stronger gank.

Beyond that, it really depends on personal playstyle. With max CDR (something you should strive for on Fiddles), Drain has a 6 second cooldown and a 5 second duration. It is your "nuke", and your go-to spell, awesome for soloing other champs. You should still max this first.

Your choice comes down to Dark Wind vs. Terrify. In years past, Dark Wind's number of bounces increased with level, meaning that a maxed Dark Wind was a terrifying spell that could bounce 15 times, silencing with each hit, and dealing impressive damage to boot. Now a days, it is limited to only 5 bounces, which makes Terrify the better skill, because it is hard CC, and Fiddles is slow and without escapes.

Max Dark Wind second if you need the help farming minion waves, or want higher burst damage. Otherwise, max Terrify second for the increasingly long CC duration.