Q 1.But why do we specifically do so in a clockwise direction?
This is because, 'Pra-Dakshina' means 'standing to the right (DAKSHINA) side of the Deity
प्रदक्षिण a. 1 Being placed or standing on the right, moving to the right; प्रदक्षिणार्चिर्हविरग्निराददे R.3.14 (sanskritdictionary.com).
If one moves around a deity keeping Him or Her always in the right side, he or she must move around CLOCKWISE.
Q 2. I assume clockwise circumambulation is a universal practice; if not, please correct me.
Yes, there are variations in the scriptural injunctions regarding 'pradakshina' in the sense that all pradakshinas advised are NOT circular. For example, according to the YAmala-Vachana,
trikoNAkArakam devi shakeh kuryAt pradakshiNam/ardhachandram maheshasya pristhatascha samiritam// meaning : For doing pradakshina of a Shakti (Goddess) form, walk forming a triangle is advised. For doing pradakshina of Lord Shiva, half-circular movement on the back side of the deity is advised.