I thought Olivia Coleman was just right, playing the Queen as she was at that time. The Queen had a different manner when younger and when older.
As for Will Carling, that was more a thing on his part than Diana's so much so that his thenwife got fed up with him mooning about her (not the only time that happened), but it didn't turn into an affair.
The heart surgeon, Hasnet Khan, was a true love affair for two years, kept very discreet. They really did love eachother and Diana had hopes for a future with him; however, he was married to his work and couldn't cope with even the idea of press/public interest, so it ended. That was in 'The Crown'. There is also a very good film, 'Diana' starring Naomi Watts I think, which is centred on their relationship and gives more detail. That is well worth watching.