Solder or flex for water heater?

We find that water has different hardness issues in different places. Different municipalities treat their water in different ways. So no two waters are alike and no material is universal.

Those copper jointers have problems with embrittlement, the corrugations temper or anneal and are prone to cracking in earth quake prone areas. If you do use them do not reuse them and if you are in a motion area I might pass on them.

I would treat an air line in copper the same as a water line. A lot of times the fact that the water has the electrical ground connected to it may or may not increase reactions between the metals. Since we can not say for certain that we would have no current of any type in the airline, we should use dielectric procedures.

Many times just the air flow motion can cause a static current, I've heard many stories of air ducting for wood vacuum systems in which they needed to be grounded in a certain way or a spark could cause an explosion from the wood dusting.

3/4 slip pipe x 1/2" male and or female copper pressure sweat fittings are available.

Copper has a few real key advantages to pex biologically. Copper is bacteriostatic, it inhibits the growth of life, like the root destroyer or a copper nail in a tree. Copper tube has an ability to inhibit bacteria, some micro bacterium and spores. It's not magic, but it is nice to not have a pipe that fosters growth of organics.
If you opened up a PVC water line you may notice that the line has an internal black film within it. This film in some cases can foster growth of organics. I would venture to say that if you have no or low chlorine levels in your water that there could be a potential for unchecked organic growth. This could possibly limit the installation of this material in some places.
It's possible that you may be able to get a variance if you have in home treatment.

I like Brigit and Canfields. Lennox works nice too. If it's in the budget use a nice strut clamp with an isolation bushing

I never touch a fluid joint to cool it, I may wipe the drip so as not to leave a sharp point but never spray or wipe with a wet rag until down to around 200 or so. Then I spray a bit ofNature works organic cleaner at it and wipe it with a scotchbrite sponge. Pretty.

For the solder joint next to the abs pipe I would use a section of tin sheet or a small sheet of temperature insulation or even a heat shield product to deflect the heat from the plastic joint. I would heat at a point away from the plastic if possible and let the heat sync towards the joint.

I personally would not use a shark bite fitting in a concealed location. As a pro I am expected to install a long term and reliable repair.
I would always work towards a properly caped, cut, sound mechanical joint over an o ring type joint. I guess I'm old school, but for me it's not that hard to give you a 40 year solution.
I would rather split and peel a tee or a cap out of a bad spot than hack it, bite it and run only to get a call back in 2 years to fix the same troubles.