Good players, generally, know the basic gardens strategies. Not buying coppers isn't going to confuse too many folks what you are gunning to get.
To the initial post:
1. That depends on how you are doing your gardens setup. Simple "gain a card" setups say you should gain as many useful cards (workshops, bureaucrats, caches, etc.) as you can easily risk before piling the gardens quick. For contested matches this may mean as few 3 or 4 card gainers. Some setups (like scheme/golem/margrave), prefer to hit the gardens as soon as the combo hits. Other shots, may encourage going for a big engine and piling out quickly near the end. For instance, KC/hamlet/council room can get you up to 16 coin (half the garden's deck) without too much trouble. On your last turn, it is easy to get 15 or more buys making gardens more cost effective than provinces. Goons engines in particular can work best if you maximize your buys and pile out the gardens in one go, pile out, and buy mad copper at the end.
2. Depends on a lot of stuff. Gold can be extremely useful if you plan to 3 pile with duchies. 5's can be great (e.g. haggler let's go garden/silver; cache is almost its own reward in a gardens game), but they can also be kinda lackluster (apprentice is worthless in a gardens game without +buy). More often I'm going to be tempted to down-buy to the gardens rather than worry about gold or silver.
3. I'd go for IGG most of the time. IGGs put curses into the other guy's deck - dropping more of his hands from 4 to 3 and the lick. Further, because each time you play an IGG you can nab a copper, your gardens will be that much more powerful. At some point I might stop buying IGG in favour of gardens as the curses are dilute enough I'll never miss a gardens; also, I won't add that much bulk from this IGG. The swap depends on if I think the IGG and gardens are being contested.