Windhelm probably offers the highest convenience factor. The market neighborhood, directly to the South, offers access to a full set of outdoor smithing equipment, as well as an Arcane Enchanter, at least three 'high-value' merchants (two general goods, including Niranye who is a Fence, and a Blacksmith), all without a loading screen. The house itself offers both Alchemy and Enchanting facilities, as well as the highest number of display racks for equipment of any home, and Windhelm offers an additional battery of merchants over in the Grey Quarter on the eastern edge of town.
Downsides include the somewhat high price, long distance from Windhelms fast-travel point, the prolonged string of quests required to buy the house (you basically must resolve the civil war), and the potential for significant bugs related to the Blood on the Ice quest, in which the house is involved.
Riften deserves an honorable mention for Thieves Guild members. While it lacks smelting services, the upgraded Ratway, with it's 4 additional merchants, puts somewhere in the neighborhood of 12,000 Merchant Gold in close proximity - easily the most of any city. Those who choose to take the moral high-ground may find the mercantile climate of Whiterun to be more hospitable however.
There is only one functional Atronach forge, underneath Winterhold.
I don't recall any reuse of the art assets for the forge elsewhere, but Skyrim is a big place, so it's certainly possible that the model is sitting around in another cave that I've forgotten about.
Best Answer
Jewelry can not be improved as the only stats they have are from enchanting. Also, once an item has been enchanted it also can not be altered, this includes quest given items. However, the stats can vary on some quest rewards, often depending on the players level.
Sources personal experience and wikia