Hi, we are nearing the end of the process of migrating from Elbrus to 2019.1. In fact, our sandbox is finished except for one service/http request problem I am having. To address your questions: (1) we had no issues installing the 2019.1 bundle, and in fact had to, along with extension and theme dev tools to then have them available on Sandbox. It sounds like you may already be beyond that, so we've also had no issues deploying extensions and assets to have them "ready to go" for when we finalize this. (2) I am not sure, our site is already fully https (3) Our Elbrus site is highly customized, so this was a time consuming project, but after the initial learning curve (documentation out there can be confusing because much of it deals with migrating to pre-2018 versions, or before the extensibility API or whatever they call it), it wasn't that bad -- but my advice is to do it all on your sandbox obviously and treat it pretty much like a new implementation, especially if your site is already highly customized.