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Firstly you cannot introduce the two hamsters, some dwarf hamsters can live in pairs but only same sex siblings that have never been apart even then once they reach maturity they often fall out, fight & need to be separated.
Introducing Lego to Gimli would be extremely stressful for both of them & would be highly likely to end in fighting, injury & even death.
It's not natural for them to live in pairs like this & all hamsters will live very happily alone.

Just take your time with Gimli, she will get over the stress but it will take time & patience.
Hissing & screeching isn't that unusual for a nervous ham so don't worry too much about it.
Was she hurt at all when the cage fell do you know? I understand you can't do a health check without stressing her but watch as much as you can for any signs she may be hurt.
To begin with just spend time talking to her quietly when you're by the cage, even if she's hiding let her slowly get used to the sound of your voice & you being around & she will learn that you're nothing to fear.
I find with a very nervous ham it can help to spend some time talking to them, leave a treat, a sunflower seed or mealworm, in the cage then walk away.
It takes time but they will then make the association between you & good things, your ham should then begin to gain confidence & you can move on with interacting, taming & bonding from there.