What would the visa experts suggest for this situation:

When I am 50 I can get a retirement visa based on bank account, but I might want to work remotely for a small US company (LLC) that I would be an active member of (much more active than an employee, more like a co-partner). The work would consist entirely of computer work in my apartment, but the website, office, legal business entity, other partner, and financial transactions would all take place in the US. There would be some customers coming from Thailand but only in the sense that web based customers come from anywhere.

While technically "verbotten" from the no work in Thailand on a retirement visa rule, would this practically be any problem at all?

Obviously, the US taxing authority would be aware of the work, but I don't see how Thai immigration would know about it or even care about it as the activity doesn't involve the labor market in Thailand at all. I also can't think why Thai immigration would be interested in examing US tax records.

Is this a fairly safe option? If not, is there any less underground way to do this, visawise? From what I have read so far, I don't know of any short of becoming a Thai company rather than a US company. This kind of thing, digital international work, is obviously becoming more common, and countries are not quite up to speed on how to deal with it.

Under current policy it should not be a problem.

lopburi3 is right as usual.

Your question sound like one asked in law school.

It is the pentultiment question in a chain that starts with working for a Thai employer in Thailand and is carried to the extreme remote logical end.

No work contacts in Thailand and no payment in Thailand.

Lets explore what is work. Many expats invest in many endeavors including stock markets for which they make money and "write off" investment expense against capital gains received in other countries, or not at all in the Channel Islands. Is this work?

We sure work at it!! When does making money become "work"?

When the government tells us that endeavor is work. So far Thailand hasn't made an issue of it, probably because enforcement would be almost impossible.

While in the drug enforcement area, I would suspect that there is inter-govermental information sharing as to assets, income and money flow, in the legitimate area of money making endeavor, I think it is extremely remote.

In any case, if they ever come down on you, the definition of what is work is subject to interpretation and would not be criminal, as you have a reasonable belief in the legality of your activity. Thus regulatory penalties would be all you woujld be facing, in my opinion.

Take paying taxes as an example. Failure to pay taxes based on a reasonable belief that they are not owed exculpates one from criminal penalties, but not financial ones. Thus, I have always proceeded in deducting expense until such time as I am told it is not a legitimate deduction. Why hamper yourself until your told by competent authority is is not permitted?