Try angle iron brackets instead of just flat. 1 inch angle iron notched at the bend and welded may be stiff enough. Flat metal does not have all that much stiffness until you get to VERY thick pieces.

First off, while the design technically could work, I have to ask you how many people you plan to sit there? (You might get two.) And why not cabinets at the back wall? (No legs occupy that space and it's lost storage.) Do you really think that someone will really occupy a space to stare at a wall? (I don't know of anyone who would voluntarily sit there. They'd rather stand and be near the cook.) It won't be very social. (Seating areas are usually meant to be social gathering spots and this won't be.) I'd challenge you to post your entire design as I think this is quite problematic from a function and flow standpoint.

Our raised bar on the peninsula will be 18" wide. About 5" of it will rest on the supporting wall; about 1" will overhang on the counter side; and about 12" will overhang on the other side where bar stools will be placed. It will be granite and therefore pretty heavy - its 7' long. We didn't want corbels to interfere with seating, so our granite fabricator is routing the underside of the granite and installing 1/4" steel plates under the bar (every 18" or so) as supports. The routing allows the steel plates to be flush with the underside of the bar. They will be visible only if one bends over and looks up underneath the bar. They charged nothing extra for doing this.

You should do the calculation yourself to confirm. I would assume 3.5" from the front edge of the countertop to the front of the sink. You probably wouldn't be able to get it to be much less, maybe 0.25". You also need to look at the measurement requirements for your faucet. You will need a bit of clearance between the back edge of the sink and the faucet cutout hole. You also need to look at the exact faucet and make sure that the handle doesn't have extra space requirements. Some faucet handles extend behind the main tube -- if you are short on space, make sure you have one that doesn't force your fingers to bang against the back wall. If you have a backsplash behind the sink, you should deduct the width of that material from the overall space requirement. If your sink is 18" front-to-back you should be able to find a faucet that works, but it will be close and if you pick the wrong one it could definitely be an issue. I would try to find the right matching faucet rather than extend the depth of the countertop, which can lead to other issues (e.g. harder for people to reach the faucet or anything behind it).