Yes, sending 10,000 rows from HTML to Omnis window via URL is probably not very likely to happen, I imagine sending messages but certainly not anything big.
Sourcing HTML page, this could be done in two ways, initially using Omnis as web server to facilitate Omnis side development and test, eventually serving pages using third party web server. The idea is to have the HTML work both ways without changing any code but simply changing URL on oBrowser.
As to reading data from Omnis, the RESTful connection from within HTML page is what I would do. Question is how to structure Omnis side to provide RESTful services.
I was thinking about creating several remote task classes each taking care of the particular resource rather than having all api methods published in the same remote task class.