What's the difference with "allan" and "ma's"? Are they interchangeable?
They mean the same but you'll hear "allan" in the North and "mas" in the South.
Allan is used on all the signs to say exit in the south. Do we only use it in that scenario down here?
I live in North Wales so can't say for sure, but based on TV dramas set in the south, it would seem that "ma's" is usual in speech. It may be that "allan" is considered more formal in the south and therefore used in writing and for signs.
Would 'mynd allan' cover both going out as in leaving the house to go outside and going out as in going to a pub or restaurant (I'm going out tonight)?