By looking from your scenario, I think the gadget SIM is the best for your mum. It's RM10 (internet sharing) + RM10 (gadget SIM) on top of Ambassador 68 monthly bill. By the way, may I ask how's your mum use internet? If only use for WhatsApp, I think you may just upgrade her to latest the prepaid plan, no need to have two SIMs.
I don't think gadget SIM can be used for WhatsApp as it can only be used solely for internet. But I could be wrong though.
Nah, too much work for you.
Did you read Mr Gray's post?

Yes, she is using it for WhatsApp as she just started to learn how to use a smartphone and later on Facebook. The thing is if I upgrade her to the latest prepaid plan, she may lose her RM 38 super long life which is valid now until July 2018 as we just did it for her two months ago.

If Gadget SIM cannot be used for WhatsApp then I will need to find another alternative.

Maybe worse comes to worst, gotta register her a supplementary line under any of us or a new Ambassador 38 for her. Does anyone know if her existing credit can be transferred to postpaid as advanced payment?

Would like to know whats the quota top up charges are. Planning to replace unifi if it's worth it

You can convert to prepaid first and change to Ambassador 38 after that, it can be done one after one at the same time. It works and also depends if the staffs are willing to help you to do or not.