+1 to the previous post. The problem with loosening a steel, cad. plated steel fastener from aluminum is that they "marry" over time. Moisture, oxygen and other corrosives cause the differing materials to become an amalgum of the materials which bonds the stud to the aluminum. If there is any significant amount of stud locking material left from the factory assembly (assuming they used which I doubt), it will aid you as it tends to exclude corrosives.
As said, heating is the trick and you need not be concerned with over heating by a mini-butane torch. The problem is that heating needs to be done very rapidly because simply making everything uniformly hot has only a small benefit in releasing the corrosion seized fastener. The trick is to heat the stud to red heat, very quickly. This accomplishes several things, the most important of which is to cause the steel stud to expand more quickly than the aluminum. You need quick expansion and high heat because you want the steel stud to become plastic as well as to be prevented from expanding by the aluminum threaded hole.
A steel stud, heated into the plastic range tries to expand radially (become bigger in cross-section) but, if restrained by the surrounding aluminum hole, the stud cannot expand. Since the expansion pressure due to the expansion is so great, and the steel heated into the plastic range, the steel begins to flow axially (lengthwise). In this way the steel stud becomes longer but since the amount of material within the stud remains the same, the stud, when cool, is reduced in diameter.
A smaller diameter stud has more clearance and usually released from the hole with little effort. Welding a nut to the broken stud accomplishes this as well as to provide a means of turning.
I recommend using a larger torch such as a propane plumbers torch to rapidly heat the stud. The mini-butane torches are OK for soldering smaller items but lack the BTU's (quantity of heat) to rapidly heat an object of this size effectively unless you are fortunate. I always reach for the larger propane (Bernz-a-matic and similar) rather than my little butane, FWIW.
If it's a stubborn one I run it up to a friend's machine shop and use his oxy-acetelyne. It's not worth the space for me to keep tanks for mine these days as do so little bigger stuff and the propane works well for most.
Get a propane torch onto the stud, heat the snot out of it until it's red hot, let cool while melting a parafin wax candle against the threads as close to the aluminum surface as possible. Give the end a few solid smacks with a hammer to help loosen but don't go crazy as you want to loosen the threads and not drive the stud deeper. Hard taps with a small hammer rather than heavy blows with a large one is the ticket.
Clamp on you 10" vise grips, stud extractor or other holding device and work it loose. If it won't move, heat again, wick in some candle wax, tap and try to turn. Try to avoid loud use of "technical language" as it frightens the children.
I used to obtain supplies of old castings and components with siezed fasteners as practice for training new technicians in the college. They were not happy but learned to remove seized and broken such that they were never troubled again by the issue. Fork leg studs are not the ideal beginning but that's what you got.
Keep in mind that the hardware guy likely works there because he knows a bit about hardware and that sort of thing but isn't a machinist or tech. He's likely doing his best to help you with what he knows so be grateful and polite. Neither of us could likely step in and do his job either.
None of that makes him an idiot or deserves the slander. Let's play nice and share, folks.
