Translation:Turn right by the hall.
In these types of sentences, can wrth be changed to yn ymyl or ger?
Yep, those would be fine too.
Why is wrth used here in lieu of mewn?
'wrth' means 'by' in this sentence, while 'mean' means 'in', so in giving directions you would tell someone to turn when they reached the hall (ie 'by the hall).