A feature that I miss is that if my mum does a transaction that is over a certain amount, it would be a very cool if I could "co-sign" it.
It's not possible unfortunately. You would have to co-sign every transaction and you would be limited to a small amount of wallets which support multisig.
So if it was your mum you would recommend electrum?
I would go for the
Green Wallet since I didn't experience any problems while using it, but it might be easier for you to explain her how to use the wallet over the phone if you choose Electrum since you have used it. Green Wallet's UI is much more appealing in my opinion.