I will soon commence evaluating an increase of my investment in a Thai registered private limited software development company, in which I am presently a minor shareholder [appx. 0.5%] . . . I expect to be in and out of Thailand a number of times over the coming 4-6+ months to conduct due diligence, and if favorable, execute the increase in my investment. The time in Thailand I will need to complete the evaluation and negotiation cannot be accommodated with a tourist visa, which I previously relied upon.

What kind of visa is best? . . . does there exist a multiple-entry non-immigrant business visa for this purpose? . . . I would like a 1-year visa for this purpose . . . what are the possibilities here?

I will soon commence evaluating an increase of my investment in a Thai registered private limited software development company, in which I am presently a minor shareholder [appx. 0.5%] . . . I expect to be in and out of Thailand a number of times over the coming 4-6+ months to conduct due diligence, and if favorable, execute the increase in my investment. The time in Thailand I will need to complete the evaluation and negotiation cannot be accommodated with a tourist visa, which I previously relied upon.

What kind of visa is best? . . . does there exist a multiple-entry non-immigrant business visa for this purpose? . . . I would like a 1-year visa for this purpose . . . what are the possibilities here?

I would have thought a multiple entry non-Imm "B" would be the way to go: up to 90 days on each entry, valid for entry up to one year from the date of issue. You don't mention where you will be applying from - multiple entry non-Imm "B" visas appear to be easier to obtain in the US, UK and other countries "far away from Thailand", whereas they are much harder to obtain closer to home. Most SEA embassies won't issue them period.