[QT-Pro]

the only options I get after a long press are: 
Edit Source
Make Default 
Remove Source 
Choose Thumbnail

That's because you've made that folder a source. You need to go back a level and then add that as the source so when you enter it you can see the folder you want to copy.

Thanks for clearing that up, Hitcher. Indeed you are right, I had made it a source.

[QT-Pro]

I will have to check whether all those prerequistes have been enabled. I think they have but I need to go home to see that. 

In your 2nd screenshot, there is no "Install from zip file" option. Where will I find that please, and are you sure that installing this default Estuary skin will not remove all the extensive customization I have done of the original Estuary skin?

It probably will remove some, not all, but you can always return to your original skin to see your changes and do them again in new version. You'll have 2 skins to choose with which one to work.
To se "Install from zip file" in Home screen scroll down to "Addons" and it's the third option in top icon row. Or go to Settings -> Add-ons -> Add-on browser and choose Install from zip file.

Btw, after installing a new skin you'll need to reboot before it becomes visible in Add-ons -> My add-ons -> Look and feel -> Skin. There you have to long press on your new skin and choose Information -> select "Use". After this you go to Settings -> Interface -> Skin and there finally select your new skin.

As expected, your proposed solution worked perfectly although it was actually simpler than you had imagined because, as soon as the skin had been installed from the zip, immediately following reboot, Favourites had become my start page without having to select the skin. None of my modifications had been lost however, curious as to how the new version of Estuary would appear in Interface > Skin, I took a peek in the drop down. There was only one Estuary so it would appear that all the mods I had made were not saved in the skin folder itself but in other areas of Kodi. 

As this post concerned how to copy and paste, which was adequately explained earlier, and the modified version of Estuary was supplied to me by vpeter in the CoreELEC forum, I will provide more details in my original request there. Suffice to say I would like to thank both you and Hitcher for all your assistance. Although I have not learned how to use SSH, I have definitely learned a lot more about the use of Kodi.

[Hitcher]

You're welcome.

Thread marked solved.

[Sholander]

As expected, your proposed solution worked perfectly although it was actually simpler than you had imagined because, as soon as the skin had been installed from the zip, immediately following reboot, Favourites had become my start page without having to select the skin. None of my modifications had been lost however, curious as to how the new version of Estuary would appear in Interface > Skin, I took a peek in the drop down. There was only one Estuary so it would appear that all the mods I had made were not saved in the skin folder itself but in other areas of Kodi. 

As this post concerned how to copy and paste, which was adequately explained earlier, and the modified version of Estuary was supplied to me by vpeter in the CoreELEC forum, I will provide more details in my original request there. Suffice to say I would like to thank both you and Hitcher for all your assistance. Although I have not learned how to use SSH, I have definitely learned a lot more about the use of Kodi.

I'm glad that all worked out for you; my extensive explanations were made in case of coping a needed folder, but this is how it should work when you install an addon from zip file.
Btw, you could have saved yourself all this correspondence if you took @vpeter's advice in the first place, and just installed the zip file.., but then, this way you learned something that will come in handy in the future. Cheers