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Don't worry about that. It's piped the way it's piped, and you can make it work the way it is.

The bypass is there to prevent too-cold water from returning to the boiler, which could cause thermal shock to the cast iron and risk cracking it (which almost never happens anymore) or could cool the boiler's exhaust gas too much, causing water vapor to condense in the exhaust stream, which can corrode the inside of the boiler.

So the mfr typically requires a bypass valve which returns some of the hot supply water back to the boiler, raising the temp of the return water.

I have two boilers where the mfr also required a bypass, but none was installed, and 30 years later the boilers are still fine. So the necessity for a bypass in some situations is debatable. The good news is that your installer did what the mfr required and installed one.

By closing the bypass valve, you force all the hot supply water to go out to your radiators, where you want it. And since your supply water is already getting fairly hot, it will still be fairly hot when it returns. I'm not an expert, but I think the rule of thumb is that as long as the return water temp gets up to 130 within 10 minutes of the boiler starting from cold, that's fine. My two boilers take even longer to reach 130 return (like 30 minutes) and as I said, they're still fine after 30 years.

To answer your question about the 0 psi, when the boiler gauge shows 0 psi, the boiler and all your pipes and radiators will still be full of water. The only thing that will have lost water is the expansion tank, because as you drain the boiler down, water from the pressurized expansion tank will be forced into the boiler to replace the water you drain, so the boiler and all the pipes and radiators will remain full. So by opening the boiler drain valve, you're actually draining the expansion tank, which is the ultimate goal.

Just follow Ed's instructions and all will be well. 😊

find a service provider you can trust.
your going to need one sooner or later!

Based on what I've learned from being on this site for awhile and as a fellow homeowner, I think you want to figure out why it got sooted up like that.

looks good is not troubleshooting

You need a combination analysis to tell!

I'm not a heating pro, just a homeowner/mechanical engineer with a similar expansion tank setup that had air management problems that I was able to fix after learning a lot from the helpful heating pros here.

No doubt they will have good advice for you. Meanwhile, I see a few obvious things:

1. Boiler pressure gauge reads 30 psi at 140 degrees. Most residential boiler pressure relief valves are set to relieve at 30 psi, so you are perilously close to popping the relief if that gauge is correct. If your system was set up correctly, it shouldn't be above 20 psi or so at that temperature.
2. You expansion tank setup is going to allow dissolved air to migrate from the tank back into the circulating water, where it will end up in your radiators and require regular bleeding. This can be fixed by replumbing the expansion tank with an Airtrol fitting, but that may not be your priority right now.
3. If the 30 psi gauge is correct, it also means that your tank is probably waterlogged, ie too much water and not enough free volume left for the water to expand into. This can be remedied by draining some water out of the tank to free up more volume for expansion, but it does take a few specific steps to do correctly.
4. The ease with which air from the expansion tank can migrate back into the circulating water, plus the apparent water-logging of the tank, all suggest air management problems that may be interfering with circulation.

Even if it turns out that the root cause of your problem is something else, your expansion tank/air management system could be improved if you want to not have to bleed your radiators regularly.