1. When someone says "all things", they could have also said "everything which is", or "all things which are".
"I like everything which is beautiful."
"I like all things which are beautiful."
"I like all things beautiful."
I can't think of another situation when I would say "all things", but one might exist.
2.
All three of the sentences are wrong, the three sentences I think you want are:
"My mother is just as beautiful as when she was young."
"My mother is beautiful, just as she was when she was young."
"My mother is as beautiful as when she was young."
The first and third are different from the second in meaning.
The first and third ones mean her beauty is equal to her beauty when she was young.
The second one means she is beautiful now, and was beautiful when she was young, but not necessarily equally beautiful.