Don't use the 12/2 line.
What it's basically saying is that you will have two stats, one for propane and one for the boiler and a two stage stat would probably be the best way to go. In this case the boiler can supplement propane, because both can be on at the same time.
But based on what the boiler Co wants you to do is to is to have two stats. One for the boiler and one for the propane. So, lets' explore this a bit.
The limit switch has Nothing to do with what goes on. What the limit switch does is turn on the fan when the gas is hot enough and turn it off a bit later to increase efficiency. Some furnaces use timers and sometimes it's done in the stat.
R to W says Call for heat
R to G turn on the fan
You do have to MAKE SURE that your furnace will operate the fan by itself in the heat mode. Some tstats and furnaces will not do this.
What it's saying is, if there is a call for heat on the boiler stat, turn on the fan in the furnace. This also assumes that the propane stat is off.
A better way would be a 2 stage stat, but it may involve extra wires that you may not have available. It could involve at least 2 (C) or common and W2 ( second stage heat).
One of the Honeywell 8000 series stats will work. I'm not sure exactly which model, but you need a 2H/1C at least. Prices range from $100 to $200 depending on model. If this is a vacation home, then you could consider stats that can be controlled via the telephone or internet.
The Vision Pro stat has way too many options, so you might get lost configuring. Furnace Filters, Air Conditioner Filters, Air Quality - iaqsource
has manuals and the stats in an easy format to look at.
One stat (the most expensive), uses only 3 wires to connect to the furnace.