Use whichever is easiest for you to get that meets the BOI requirements. You're not required to use that insurance if something happens, it's just a visa requirement.
When you get approved, you will receive an endorsement letter. This letter is what you use to apply for your visa. The letter is only valid for 60 days.
You have 2 options for obtaining your visa, and you must choose only one, and you cannot change once you choose.
1) Apply to your local Thai embassy through the e-visa system. This will result in you paying more than the 50,000 baht that is the official price at most embassies, because they set their own exchange rate. It also means your visa will be a pdf document that you will need to print and carry around for 10 years. Most embassies will not process an application if you indicate your travel is greater than 3 months away. Since your endorsement letter is only valid for 60 days, you need to apply within that time and indicate that your travel will be within 3 months from the time you apply. Once you have the visa, there is no requirement to enter Thailand. The visa will be valid for 5 years from the day it is issued. You can enter any time within those 5 years, or not at all. The choice is up to you. Shortly before the 5 years ends you'll need to re-qualify for your 2nd 5 years. No one knows what this process entails yet.
2) Travel to Thailand and make an appointment with the BOI to pickup your visa in Bangkok. This will result in you paying exactly 50,000 baht directly to immigration, and you will receive both a 5 year and 10 year stamp in your passport, so there is nothing extra for you to carry around. If you won't be in Thailand within about 45 days of your endorsement letter being issued, then this option would not be available to you as the letter would expire before you could get the visa and you would have to apply all over again.
Visas are meant to be obtained from embassies before traveling to a country.
While some immigration offices do allow you to apply for certain visas inside of Thailand, you must still meet the requirements of those visas, which including proof of finances which is always in a Thai bank if applying in Thailand.
No bank in Thailand allows accounts unless you're on a long-term visa.
So your option now is to leave Thailand and apply for a non-O visa at a Thai embassy. You don't have to go to UK, you can go to Laos, or Vietnam, or some other country and apply. Embassies accept financial proof from any bank, not just Thai banks.
That depends on your immigration office. Most want a new TM30 when returning to Thailand, but some are fine without it if returning to the same address.
And legally your friend needs a TM30, but realistically unless they'll be going to immigration for something while they're staying with you, it's not necessary.