As per MB under severe conditions needle should not go to the red mark.
Q-1 Answer Yes if outside temp is high the heat transfer is low and engine runs at higher temp.

Q2- When you get a chance change the coolant with a good mixture. I used the red Texaco coolant and I am happy with it. Also using tap water is no good. It has salts that will deposit with time. I use distilled water or a very good quality drinking water gallon from grocery store.

Q3. I always carry xtra pair of top and bottom hoses on all my cars. Just in case.

On the thermostat issue its nothing but MB OEM also if its not broken dont replace it. I have seen horror stories with replacement thermostat.

Make sure aux fan works. Also turn off the AC occassionaly during the trip. I usually do this like once every 2-3 hours drive for 5 minutes. Check the main cooling fan clutch. Should be hard to rotate with hand when the engine is hot( but switched off LOL).

Good luck

MVK

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1985 300D Turbo 179,000miles