Switching from TIER1 to TIER 2 - visa expiring

[WarisAli]

Please can you guide me,

I wish to switch from TIER1 General to TIER2 and my visa expires in the next 20 days. I am giving two interviews next week.

If I am selected in any of these interviews, and offered a job and a CoS too, but if my visa expires, what choice so I have, can I write to the Home office and ask for an extension and apply from here, or do I have to go back and apply from my country. In that case would the employers need to do a market test again as there is no time now for a market test and switch while in UK.

If l leave UK and my sponsor advertises for the market test, and I pass that , can I still apply for TIER 2 from my country? Do I need to show £800 maintenance funds here in UK that I have held since last 3+ months.
I checked all points and I have them for language and education and the skill within the selected TIER 2 band as well.

Sorry if I have worded it in a long manner, please can you advise me ,

thanks
Waris