Is one connection sufficient as lightning protection?
Lightning protection is by definition the sum of all measures, divided into external and internal lightning protection. The equating of lightning protection with a lightning protection system has long been outdated.
The internationally harmonized lightning protection standard series IEC 62305, German classification DIN EN 62305 (VDE 0185-305) and in Austria the - except for the national foreword - identical ÖVE-EN 62305, apply only to buildings with lightning protection systems.
Conventional lightning protection systems must have
at least two down conductors; only one would be non-compliant with the standard.
The effect of grounded gutters is likely minimal. However, I would say that they definitely belong directly to the foundation earth.
The integration of gutters and the base of downpipes is mandatory in lightning protection systems, but otherwise optional.
If you absolutely want to ground your downpipes now but can no longer reach the lugs, you could instead drive one ground rod each into the ground. Pay attention to material pairing due to contact corrosion.
Separate ground electrodes totally contradict the basic principle of equipotential bonding and are dangerous and non-compliant with standards. Even under very old ABB regulations, they were only permitted under strict restrictions exclusively for lightning protection systems, but have always been prohibited for antennas.
Equipping rainwater downpipes with their own ground electrodes is less dangerous than pointless.
Yes, you understood correctly, without an external system you can leave the gutters as they are.
Agreement, since "external system" means external lightning protection.
Equipotential bonding is the technical term for the grounding of the house.
Objection!
Since I currently have my "house standard" at hand, I copy the official terms according to IEC from it:
3 Terms, symbols and abbreviations
3.1.8 Earthing system The entirety of electrical connections and devices used for earthing a network, an installation or a piece of equipment.
[IEC 60050-195:1998, 195-02-20]
3.1.10 Earth electrode Conductive part embedded in the earth or in another conductive medium, e.g. concrete or coke, which is in electrical contact with the earth.
[IEC 60050-826:2004, 826-13-05]
3.1.13 Equipotential bonding Establishing electrical connections between conductive parts to achieve equal potential.
[IEC 60050-826:2004, 826-12-01]