But how is mens rea enforceable in the buying and selling of acids?
It is the person buying- why are you buying it ? Plausible or implausible.
Are the companies who sell acids complicit in the mens rea?
No - it has a perfectly legitimate use - as do many legal but potentially dangerous items. Selling to under 18s would be an offence- same as alcohol, knives and tobacco.
How can we be sure that the girl was carrying it with intent? She may well have had it put into her bag by a relative
Oh come off it. In the unlikely event of that happening she would give that explanation.