Following the earl's second marriage were the late Princess Margaret and the former Lucy Lindsay-Hogg known by the title Countess of Snowdon during the Princess' lifetime?

Essentially, yes.... Princess Margaret would be styled "Her Royal Highness, Princess Margaret, Countess of Snowdon" while Lucy Lindsay-Hogg would be styled "Lucy, the Countess of Snowdon" (seeing as if Lindsay-Hogg and the Earl of Snowdon where still married). Seeing how they are now divorced, she'd be known as Lucy Lindsay-Hogg, Countess of Snowdon as in past tense (like Princess Margaret, Countess of Snowdon)