External Visarga Sandhi + unvoiced 3.4.1

Question 1

Any visarga before unvoiced sounds

Answer 1

Regardless of visarga comes from s or r ending, it will remain visarga
muneḥ śakti -> muneḥ śakti (from final s)
rākṣasāḥ karśanti -> rākṣasāḥ karśanti (from final s)
punaḥ punaḥ -> punaḥ punaḥ (from final r)

Question 2

Any visarga before unvoiced sounds: exception

Answer 2

When the following sound is a UPSV (unvoiced plosive of sibillant containing varga c, ch, ṭ, ṭh, t, th) visarga changes to the plosives homorganic sibillant ś, ṣ or s
buddaḥ cīvaram -> buddhaścīvaram
agniḥ ṭīkā -> agniṣṭīkā
bhikṣuḥ tanoti -> bhikṣustanoti