Stuck in UK

Hello,
I am a doctor from Pakistan and presently in the UK on a standard visitor visa. I came here for the registration with the General Medical Council (GMC). Moreover, I have signed a contract with one of the NHS trust and my employer has given CoS. unfortunately, the day I submitted online application and planning to go back to Pakistan to get a visa, Pakistan closed its border for all incoming international flights. Can you kindly reply to the following queries

1-I came to know through GOV.UK website that under these unique circumstances we can apply form the UK to switch to
the long-term visa until 31 May.
2-In addition, how can my dependents, for example, my wife and son above 18 years ( financial needs of my son are met by me and he
is currently in the UK on Tier 4 visa) who are in the UK apply for the long-term visa.
3- Do I need to pay if I apply for a new application or need to cancel the previous application, however, I have already paid my "visa fees
and immigration health surcharge" (IHS) and I have proof of the payments.
Regards