Well, there has been a further development. A man who was the brother of the suspected aunt in this scenario (so would be my uncle) has done a DNA test too and has also come back as a close match - albeit not as close as his sister.
So I now have the following results:
BC (Suspected FC1R) and I share: 7.7% of our DNA, 543.2 cM, 25 segments with the largest being 71.1 cM.
PW (Suspected FC) and I share: 16% of our DNA, 1,130.7 cM, 37 segments with the largest being 88.1 cM.
PI (Suspected Aunt) and I share: 30.4% of our DNA, 2,153.5 cM, 41 segments and the largest is 166.3 cM
DP (Suspected Uncle) and I share: 22.6% of our DNA, 1,599.6 cM, 38 segments and the largest 136.9 cM.
Further to this, the man we suspect who may be my half-brother (by proxy of all the circumstantial evidence) is also doing a test and his result should be available this week.
What I am curious about is the DNA testing. I know DNA ranges can fluctuate anyway naturally, depending on the nature of the relationship and the sex of the person involved but both the potential aunt and uncle either come close to the 25% DNA / 1,750 cM threshold - or exceed it; however, their father died 10 years before I was born, so there's no possible way they and I could be the half-siblings. As such, if the assumed half-brother comes in around the same percentage, presumably there'd be no way from those numbers to distinguish the nature of all our relationships other than circumstantial evidence - so is there any kind of DNA test that he and I could take that would definitively tell us whether or not he and I are cousins or half-brothers?
Unfortunately his/our father is dead, so it's not an option for us to use him as a basis for comparison. My mother is dead, so I cannot get a sample of her DNA to use that to distinguish, and his own mother is completely in the dark about what is happening as they're concerned it will upset her - so she's out of the picture too. Is there any way to know for sure if there's no other way of obtaining DNA which would help establish specific differences?