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That looks more like a very minor leak thats causing the corrosion.

I am not in favor of bushings they tend to leak. I would come out with a short brass nipple with a 3/4 x 1/2 brass coupling.

And alternative is a copper female x brass male union.

What ever you do swest to the fittings before you screw them into the coil. Don't screw the fittings in and sweat to them you will cook the pipe dope and the coil will suck up the torch heat making soldering more difficult.

Why not simply put a 3/4 x1/2” brass bushing on the coil. Then use a 1/2” cu. male adapter?

You’re making it much harder than it is.

You should also have unions as close to the coil as possible and a valving arrangement for cleaning. Get a tankless valve kit, a couple of bushings and brass nipples, and you’ll have everything you need to do it right.

Hi, I'm not answering your question directly, but do have a suggestion for preventing further corrosion and making servicing easier. I'd consider coming from the coil in plastic lined steel nipples and then brass unions. From the unions, go to copper. This approach puts distance between different metals and makes access simpler, should things ever need servicing. Here's one brand: https://api.ferguson.com/dar-step-service/Query?ASSET_ID=1394252&USE_TYPE=SPECIFICATION&PRODUCT_ID=1302318

Yours, Larry

Typically on a steam boiler, there would not be a PRV before the BFP unless the standing pressure was extremely high, say over 80 PSI.

A dual check, vented backflow device. Usually there is a pressure reducing valve combined with it, if it is used as a boiler feed?

It is an easy replacement at the backflow preventer has union connections at both ends. Just buy the same model and replace it. No soldering needed.