As a monumental type , Portal Tombs , which Trethevy is obviously one as it bears all the hall marks and they are clearly not “slight similarities to other structures “ . As I mentioned earlier “Have a look at any reasonable description of the characteristics of a portal tomb and you will find that Trethevy is a perfectly logical ideal candidate in it's morphology as well as belonging to the iconic monument type of Cornwall .” points , which you didn’t respond to .
Due to the architecture these monuments have been easily disturbed particularly from animals and humans who have taken the internal deposits and external cairn material .
Many have survived disturbance and those that have show that human bone inhumed and cremated was a common deposit ,e.g. Poulnabrone had a minimum of 26 interred individuals . Of the nearby monuments of the same type , Zennor had bone deposits and Borlase had a drawing of an obvious mound ,Sperris had cremations , Mulfra vestiges of a mound .
Stone circles are not always circular , Hill Forts are not necessarily defensive , Rock Art is not necessarily “art “ if Portal Tombs are not tombs they are pretty close but more importantly that is what they called . It may not be ideal but it is a more appropriate and descriptive than “Solar construct “ .