Can the simple past also be translated a a progressive? I.e. in this case “I was looking at the clothes”?
No. The imperfect is usually better for that - Ro'n i'n edrych ar y dillad
If something happened while you were somewhere, then you use the imperfect for the continuing action, and the simple past for the one-off event:
- Ro'n ni crwydro o gwmpas y Maes ddoe pan gwympodd y stondin 'na - We were wandering around the Maes yesterday when that stand fell over.