I am surprised to see a bit of "old school" style rust staining in the photos. With modern vehicles and antifreeze formulations, very little corrosion should take place. Has it ever been run with plain water ?
The maximum cooling system pressure is governed by the pressure relief valve in the expansion tank, and hoses and their connections should be easily able to withstand that pressure. Is the hose coupling faulty ? Was there a rise in pressure so sudden that the expansion tank couldn't keep up ?
The main reason for changing a heater matrix (long job) is a leak, which is obvious from damp patches, a characteristic antifreeze smell in the cab and a greasy film forming on the inside of the windscreen. The other reason is blockage from sludge, which is rare these days if the antifreeze is maintained.
No heat is more likely due to an air/vapour lock, possibly due to a fault elsewhere. Another less likely possibility is a fault with the air distribution system in the cab. The heater matrix runs all the time (flow and return hoses should be hot), the hot air is just shunted out if you select cold.
Don't place too much reliance on the temperature gauge reading. The software is coded to show exactly centre scale for anything within an acceptable range, say 80 to 100 degrees C
