On the point about 2nd cousins.
I have 3 2C matches on MyHeritage at 3-5%, respectively 212.5, 334.7 and 316.4 cM and a fourth on FamilyTree DNA at 206 cM.
I have none on Ancestry where a 339 cM match can only be a half 1C
339 can very easily be a half first cousin as too a second cousin, as too a first cousin once removed. I am not sure where your statement of 339 is coming in when referring to second cousins or 1st half cousins? " I have none on Ancestry where a 339 cM match can only be a half 1C" ? ?
I share over 500cms to both my first half cousins ( 7% and 8%) and to my full cousins who have tested , 12% and 15% . With my highest second cousin I share 363 ( and 5%) all my other second cousins are 4% and vary between low 200s and high 200s
It is totally random the way dna is split outside that parent and child relationship, a few of my cousins , my own child is almost matching me with cms ( and he 100% not double related to them but I have basically passed down the full wack to him on some lines)
Tracy had said her known second cousin is 7% from her mum's side ( its her dad's
side that she is querying and not her 7% known cousin on her mums side, its just that I have picked up on that near impossible 7% for a second cousin)
The 7 % on her mums side I personally think is way off for second cousins, not 100% impossible of course but 7% extremely high for second cousins to be sharing , and just maybe Tracey has mixed up first cousin once removed with saying second cousin( ?)
If we go by percentages, that 7% is very likely in the 500s cMs and it is a very , very low chance of her known cousin being her second cousin. I think It's more than likely that's it her first cousin once removed and not a second cousin.
My mother's sister thinks I am her second cousin , I haven't the heart to tell about her terminology as she's elderly. A few people in my family tend to say second cousins when they're referring to their first cousins children , first cousins once removed .
Kind regards