This route for elderly dependent relatives has been virtually closed since the changes to the Immigration Rules in July 2012. In order to have any chance of success you would need to prove that your mother needs daily care for basis tasks such as dressing, bathing and preparing food. Also that this care is either not available in her home country or is unaffordable.
It isn't permitted to switch from visitor to any other category from within the UK.
You also have the added issue that she has no access to NHS treatment and the use of any NHS facilities will be charged at 150% of the cost. A serious health problem could result in charges in the high thousands.
If you apply for an ADV are are refused, it will close the door to any future visitor visa applications, as will any unpaid NHS bills over £500
How are you meeting the costs of her health care at present
https://www.jcwi.org.uk/policy/adult-de ... -relatives
Also note: This is for applications submitted outside of the UK
The Entry Clearance Officer (ECO) must be satisfied that the applicant will be
adequately maintained, accommodated and cared for in the UK by the
sponsor without recourse to public funds. If the sponsor is a British citizen or
settled in the UK, they must sign a 5-year undertaking to that effect, at the
entry clearance stage.
AND
EC-DR.1.1.The requirements to be met for entry clearance as an adult dependent
relative are that-
(a) the applicant must be outside the UK;
https://www.gov.uk/government/uploads/s ... FM-6.0.pdf
https://www.gov.uk/join-family-in-uk/eligibility
