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Calculating the condensate volume is really simple. What you need is the EDR of the radiator to start. Then the radiator output is 240 BTUh per square foot EDR — so multiply to get the BTUh of the radiator. Then that heat (and it is a refrigerant problem — it's just that the refrigerant in this case is water!) is provided by condensing the steam — at the rate of about 1,000 BTU per pound of water or steam. Close enough anyway (it's actually 970). So a bit of math and you have pounds of water per hour, then get that down to gallons per hour then down to gallons per minute.

And it is astonishingly small. Actually that's one of the beauties of steam — you don't have to handle much water.

On the leaking of steam into the returns. There are really only two ways to do that — one is to limit the steam entering the radiator to what it can condense, and the other is with a trap. In the old days, with vapour systems with very tightly controlled pressure, the former was common — either with orifices or with special steam valves (which are still made) which have what amounts to a variable orifice which can be adjusted to limit the steam, but still allow the radiator to be turned down or off. Expensive. More important, either of those requires that the pressure be maintained in a rather limited band, as both are sensitive to pressure differential. A much more reliable way is to use a trap on the outlet…

Now since you are running two pipe steam, balancing demand is really simple, since on two pipe you can control the heat from the radiator simply by partly closing the inlet valve (unlike one pipe, where you can't do that). So, for your elderly lady and the rest of the house, get it all going so she's nice and comfortable — might even put the thermostat in her area if you can keep her from fiddling with it — and then throttle the rest of the radiators to get the results you want.

Later on if the budget permits (or you find that you want them) you can put TRVs on the radiators — just remember that they can reduce the heat, but not increase it.

To find a water leak or a steam leak you need to think like water. If i was water and wanted to leave this system where would I go?

Was the new boiler pressure tested before commissioned? You may have a steam leak between the boiler sections. that is tested by overfilling the boile to about 18 to 24 inches above the water line. If there is a steam leak it will show up as a water leak when the water is that high. You may need to remove some of the covers and insulation from the sections.

Another popular leak is a return pipe that goes below the floor before it comes back up to connect to the boiler. I remember installing a boiler in a church that has a rotted out dry return the went thru a crawlspace before it got the the basement. the pipe was under about 6" of sand and was unable to be seen. After the new boiler was commissioned, the water feeder was one with a water meter. After one month of operation, the feeder registered 999 on the meter and shut off the boiler. By resettingthe 999 to 000 the boiler started right up. now I needed to find where 999 gallons of water went in a month. That is when I found the rotted away return, and it was rotted for some time… more that 5 years at least.

Replace the return pipe and WOW did the fuel bill go down. (the church's board members thought the new boiler was the reason for the savings… I didn't have the heart to tell then is was anything else. New boiler was money well spent.

After doing what Ed says, I plug all the vents with 1/8" threaded plugs and pressurize the system to about 15 pounds of air with a pump. Any leaks whistle. I usually find the radiator valve stems leaking, a few unions leaking along with the sight glass unions. Combined together, these can consume a gallon of water on a large system. The work usually takes about an hour and leaves us with a tight system.

Yup. Plus enough extra to overcome the check valve.

The hope was to keep water from backing out of the boiler perhaps all the way up into the mains — but the physics of that doesn't work out either (see some of @ethicalpaul 's comments on equalizers!).

Check valves were used in some of the more complicated two pipe systems, though — you can find examples in The Lost Art — in conjunction with some remarkable gadgets to get water back to the boiler if the boiler pressure got too high — but that is a very different situation.