OK just my 2cents but... In Florida it would be a violation but on your part for hiring an unlicensed plumber and his for doing the work. The work itself is not "incorrect" but rather poorly done. 50' is a BIT far for 1/2 and should have been 3/4"...if I was doing it I would have run 1" unless there was ZERO chance of running more pipe for more hose bibs in the future. Shut off your house meter and take off the hose bib. Check out the hose bib to see there is nothing in it and open it fully and make sure the rubber gasket looks good. (IMO I always replace hose bibs....but thats just me) Turn back on the meter and run the water through the open pipe for 1 min. to be 100% sure there is nothing in the pipe. Although it is rare it can happen that when soldering a fitting the guy used too much solder and made the pipe fill with solder. This would restrict the flow greatly. Could happen at any join he made. BTW I know after the fact it sucks but never hire anyone who isn't licensed for what they are doing. I am a sprinkler contractor, I have done this type of work many times, I would NEVER do it for a customer. It would put my license in jeopardy. Good luck, Tom

You've probably never been in a home with properly sized ductwork. Don't feel bad, it's estimated that as much as 3/4 of the heating and cooling systems in the US are oversized. 2 stage and modulating furnaces are better, but I've lived in a home with both, and both can be very comfortable and the single stage equipment doesn't make it feel more dry that I can tell. BUt you are correct, multistage cna reduce the sense of dryness you might get with a less than ideal system. THe RH of hot air is lower than cooler air, but cool air can make things feel drafty. That's why radiant heat is popular. You essentially have very, very low air velocities and much of the heat energy is indirect infrared heat which make you feel warmer, similar to direct sunlight. Keep in mind too, that unless you have an outdoor rest control for hte boiler or to sped up or slow the circulator, most hydronic air handlers are single stage for heating. So the heat is no different than a furnace. Worse, as you reduce BTU's to the coil, the airflow speed is usually ocnstant, so air temps get cooler. My point being, that a properly designed and installed furnace won;t make it feel dry even if single stage. But yes, oversized furnaces and those with restricted airflow can. Vairble capacity will always make the system more comfortable, but it will cost more. Radiant heat overall is superior, but the installed cost is much, much higher. That's why it fell out of favor. Same reason drywall took over the market and plaster disapeared. ITs' primarily a matter of cost. It's also why stone and brick homes aren't built anymore and concrete homes are the minority.

There's industrial paint made specifically for pipes that get hot. Check with a paint store.

You want to run it as 3/4 inch. The good news is that it will improve water flow when you have multiple faucets open. The only drawback is that it will take it longer for the water to become hot. This isn't much of a problem if you have many points of use (there is enough use to keep the water lines warm). If it was a single faucet, I would say keep it 1/2 inch. I replaced some 3/4 inch line to the kitchen and replaced it with 1/2 for that reason.