If I remember correctly the post was in this area. One office said only 2 years but another gave 5 years. I am sure the post was very recent. Have a search.
I think it depends which Land Transport Office you go to. I saw a post where a DTV holder was told 2 years only at one office but got the 5 years elsewhere
Bangkok Bank and others used to try and sell insurance for around 5k but it has gone up recently. Their staff would strongly imply that no insurance, no account. They get a commission for the sales. However, using an agent would negate that as they get more commission from the agent than the insurance :-)
No. The 10k THB fee is for 5 years of the DTV with an entey of 180 days per entry. If you extend in country there will be a fee. If you leave the country and re-enter there will be no fee and you will get another 180 day.
With the DTV you will not be able to use a Thai legal entity. If you do any work for the inside Thailand, even if the contract was outside, you need a B and a WP