This again. Temperature rise across the furnace is going to tell a big part of what what is going on here.
Could you take a quick photo of the furnace and the burner rack?
Manufacture? Model number?
Do you have a meat or candy pocket thermometer? Drill a small hole in the return duct and in the supply ducting and see what the actual temperature rise is.
Here it is code to have a fresh air duct piped to the return air side of the furnace. When the air temperature outside is very low, it doesn't take much of this cold air to drive the temperature of the air returning to the furnace down to 40 or 50 degrees F. Now if the furnace has a 40-50 F degree temperature rise, you would only be supplying 80-100 deg F air to the home which will probably not be enough heat to overcome the heating load for the home. (Furnace runs continually)
It is also code here to have a combustion air duct from outside to the furnace room. This air is used to fire the appliances and is vented with the products of combustion to the outdoors.
If your venting for the furnace was blocked by birds (liquefied bird goo?) the burners would most likely be "rolling out" of the combustion section of the furnace and you would see evidence of this
Burners should be removed and cleaned if there is "stuff" on them. You have a tech coming tomorrow so I would hold on for him. It is actually pretty easy to do, but not getting them installed correctly could be rather disastrous!
Is there a rain cap in place on your B Vent? How are the birds getting into your venting?

So what was the verdict, Rick?

I'm in a heap of hurt now.

Measured inlet pressure 5" WC (furnace not running) and then outlet pressure ~1" with furnace running. Came back today and put in a new reg/gas valve.... same thing.

Measured inlet again and when you turn on the furnace the inlet goes down to 1".

Waiting for the gas company to show up.

Not that big of a deal. They'll put a new meter on, adjust outlet pressure and you'll be golden. I think there are some meters with built in regulator and some with external Either way, your regulator outside of the house is messed up.

New (external) regulator and back in business.

Good!
At least it kept the house from freezing up.

What I thought was bird goo he said was water and soot from incomplete combustion in one chamber. Sound right?

I didn't know it all came apart so easily to clean

Sounds right.

Very easy to get apart.

Brian 162 nailed it. It is not very often that the main reg for the home goes bad. I have seen maybe 1 or 2 in over twenty years.

The old furnaces were pretty basic and easy to work on.
The soot (carbon) build up is the worst especially if there is alot of it.
Use a good vacuum and wear a mask.
Funny I sent a reply on my i-phone but it didn't post. Who knows where it went.