This is how it goes- I ask some questions to determine the status of my prospective client ("Client"), then give Client options to take, if there is any. If it is E-visa, typically, form a company here, get a business bank account, find a place for business, contract with suppliers or whatever the business requires to be operational condition, transfer $$ for investment to US bank, register the business with US embassy in Japan, prepare documentation, then file a visa petition.
The degree of difficulty in getting E-visa depends on the amount of $$ that are put at risk of investment and how good the business plan is. The reasons for any country to invite foreign investors are because that the country can profit from business tax and that its own citizens may be employed by that business. If the business plan is sounding to serve those 2 purposes, then there is a higher chance of getting the visa.
I am not sure what you have attempted and what kind of disappointment you have had. Hopefully, we, as lawyers, will not be the reason for your future disappointment.
As long as you are a guest speaker, what you do is in a brief& temporary, and do not engage in money generating activity, and do not obtain compensation from US source, then normally Visa Waiver or B-visa would suffice. I am not expecting any bonus for my answer, no worries! but I will stop answering your question any further. If you have more questions, I suggest that you make an appointment with an immigration lawyer, like myself, and have your passport, I-94, and other letters from US Embassy or USCIS with you if there is any. And also, there are some other options and details you may want to know to pursue your goals.