On a small side project, I load the initial results with the template and then do filtering and additional searching with js. This allows me to be SEO friendly, while still having nice filtering functionality.
I am a full stack developer and am looking for a project that can generate revenue. I would be interested in learning about the business and potentially helping you if there is a fit. You can find contact details in my profile and send some more information across.
I use JS validation library + Mailcheck.js on the front end. The regex checks for format and mailcheck validates second and top level domains. Coupled with a verification mail this is good enough for me.
If you are planning to bootstrap the company then incorporate wherever you want. On the other hand, investors would be a lot more comfortable investing in a US company and clients prefer to deal with other US companies.