For the mage Kistandalius' shop, the only way to enter the back room is by paying the minstrel standing in front of the shop to recite some poetry, giving you a brief opportunity to loot. You can kill him, but since he sells charms and spell books, killing him for a single spell book doesn't make much sense.
For the potion dealer Walker, there is a trick to get into the back room, described
here and
here.
Dr Elrath's house is lockpickable until your second encounter with him. If you didn't use it already, there is a key by a rock or something just south east of the house (not sure if it works after he is arrested). After that there are a couple hacks to get back in;
FIX for the Missing Catacombs PortalTeleporting Anywhere, via hex editorIf you stand at the north west corner of the house (or any building) you may be able to toss a teleporter stone into the front room.
or you can get the potions directly
Lost Items - guide and directory of inventory files
