Is this a common piping configuration

The unit shown must have its own water circulator in it. You have a primary secondary circuit. We would need to see the ends of both sets of pipes to see exactly what is going on. I am assuming the pipes on the left in the second photo go to the wall unit.

the unit heater's coil is likely always hot when the boiler is on and it regulates temp by turning the blower on and off. monoflo needs to have constant flow in the main loop and the branch to keep flow in all the other branches.

If there is an infiltration, drafts in the crawlspace, that tends to freeze pipes quickly.

Always good to seal a crawlspace, especially the rim joist connection to the foundation.

If it is below freezing, generally the heat is flowing and pipes are safe.

The biggest concern is extended power outage in sub freezing weather.

But if the crawlspace is below freezing,invest in insulation, the gift that keeps on giving.

Well, hot goes to cold, always. Also the greater that temperature differance the greater the heat btransfer.

So insulating the space so the temperature around the pipes stays warmer will lower the heat loss from the pipe wall.

But the space is being warmed from some place/?

Either adjecent space, or heat loss from uninsulated piping. Either way the less that goes outside the structure the better. if energy consumption and comfort is the goal.

if the floor is relatively free of critters you could insulate the bottom of the floor to keep the heat in the room. most of the heat loss is likely through infiltration between the top of the foundation and the sill and the rim joist and the floor. it wouldn't be surprising at all to find an open hole that someone just covered up somewhere in construction from that era

No i would not remove the pipe insulation from the heating pipes in the crawl space. All it will take for it to freeze is an air pocket to develop in the branches to stop the water flow and then the exposed piping will freeze. I would not only insulate that piping but also box it in. Expose piping in a crawl space will not heat the crawl space enough to prevent it from freezing. The only way is to keep the water moving but circulators don't last forever either. I can't tell you how many times this is attempted and the pipes freeze. Just lowering your thermostat overnight to save a couple of bucks is all it takes.

The piping going to the left is a common monoflow piping arrangement. There should have been a lot more space between the tees. It is concerning because the pressure drop between the tee's is what allows some of the water to flow to the branches, hence the wall heater. Although the Tee's are installed with the arrows facing each other to maximize the pressure drop, the tee's being so close together might negate that. Only you as the homeowner can check that with the heating system on. You can feel if the hot water is moving.

i'm in Boston and every year we fix issues like this. Everybody wants to use the mud room, 3 season porch, or enclose there porches for an extra bedroom without thinking about how to protect the piping. i personally would never recommend putting heating pipe exposed outside in a crawl space. If you do extra precaution should be taken. I honestly can't overstate what a bad idea it is to leave pipes exposed to freezing temperatures. Also keep in mind that most residential piping systems, at least in massachusetts, are done with type M tubing, which is thin wall. only the fin tibe piping is thinner. So frozen piping will split relatively quick.

i hope i understood your question correctly

It also depends on the type of heat emitter and how far the heat emitters are away from the monoflo tee's. Thats why you need to space them apart. Yes the orifices create the pressure drop but it has to be greater than the combined piping of the supply and return and the emitter. By spacing them your allowing additional pressure drop in the main. This increases the flow thru the branches. Monoflo tees were designed with radiators in mind and they still wanted separation as wide as the radiators.