Best Of

If they are packless plug type true vapour valves, as @mattmia2 suggested and which I am rather inclined to think, they can almost always be freed up. A bit of WD-40 or some such on the shaft (PB Blaster or equivalent if you are very brave — very small quantities) and let it soak in. Then try to move the handle either way — even a little bit at first. It may take more force than you can manage with your hand (at least it does with my hand) but don't get carried away. You don't want to bend or break the handle!

Rinse and repeat. With patience they may free up.

They can also be disassembled, but try to find a diagram of one (many are in the archives here) and understand how they are put together before taking a wrench to them!

I'd go with @Long Beach Ed 's guesstimates — as tops. If the system is heating reasonably evenly and is quiet, I'd leave it alone as your charges to make it more better will eat up the savings pretty fast.

Except. Changing it to hot water is very unlikely to be satisfactory or even work, and be very expensive to boot. There is no way you are going to make back the investment on the change.

Get her to spend money on good storm windows, draught sealing, and perhaps insulation (that depends on how the house is built).

Some educated quick guesses:

The 2" bottleneck and lack of vents may be delaying the heating of radiators.

I suspect with some minimal repiping, you can save the customer 5 - 10% in fuel.

If the boiler is over-sized, which most are, and you replace it with a properly sized, properly piped boiler, I'd expect to see a 15 - 20% reduction in fuel consumption.

Converting to hot water, with baseboard radiation may save 15% -30% in fuel costs over the present usage.

You could try opening up one trap and taking the element out and run part of a cycle with the trap open and see what happens. If the issue is whatever is venting the returns is not working then the returns should vent through that trap and they should all heat or at least more should heat. If the valve is set up right and the vaporstat is working right it shouldn't let in enough steam to get to the trap, air should vent out the trap but not steam. If steam starts coming from the return that tells you that you either have traps that are leaking by somewhere or that the boiler return trap is trying to equalize the return but it can't build pressure because it is open.

It should be black iron but copper vs black iron won't be the reason it doesn't heat. It could cause some more minor problems but it likely will work ok.