Best Of

May be the most inexpensive way to go, just some pipe and a radiator trap. The existing valve, the condensate loop and the trap could all be closer the the wall. There could be a clean out valve at the floor.

You're making it a good deal harder than you need it to be. The crossover trap I suggested takes care of the venting. The loop seal takes care of the condensate.

Now that loop seal has to be proportioned to the expected maximum steam pressure, or steam in the main will happily blow through it and get into the return. This you do not want.

Now you could also go the F&T route. A lot more maintenance and expense, but as you please. Be sure to use a small enough one — the usual error with an F&T is to make it too big. There isn't all that much condensate. Note that an F&T is NOT an adequate substitute for a main vent, so you will need to put a main vent at the end of that main if you choose to go that route.

I presume the dry return is vented somewhere? Like near the boiler?

More or less as @delcrossv suggested. Use that 3/4 inch stub, right angle to inlet of standard radiator steam trap, come out of the bottom outlet of that and down to the level of the dry return without that existing trap and drop. From somewhere convenient before that existing trap, go all the way down to the floor, come back up for your loop seal and tie into the dry return with the line from the nice new radiator trap.

Use the existing trap for a doorstop.