Are your Product Backlog items "Ready" for Sprint Planning?
Product Backlog items that can be “Done” by the Development Team within one Sprint are deemed “Ready” for selection in a Sprint Planning.
Sprint Planning is for just enough design.
The Development Team usually starts by designing the system and the work needed to convert the Product Backlog into a working product Increment.
Work planned for the first days of the Sprint by the Development Team.
Analysis can best be achieved when done continuously.
Product Backlog refinement is the act of adding detail, estimates, and order to items in the Product Backlog. This is an ongoing process in which the Product Owner and the Development Team collaborate on the details of Product Backlog items. Product Backlog items can be updated at any time by the Product Owner or at the Product Owner’s discretion.
To answer your first question, within the Scrum framework it is expected that analysis occurs for items to be done in the future. In accordance with the Manifesto for Agile Software Development's values and principles, you shouldn't be creating a long term timeline with highly detailed specifications that cannot be easily changed.
Another concern, based on your post, is whether or not your Product Backlog items are scoped small enough. Are the items so large that a lot of design needs to be completed? That may be an indicator that refinement should include slicing items into smaller pieces that can be "Ready" for a Sprint. Toward your second question, this will help strengthen the Scrum Team and business relationship through more consistency; remember that the Sprint Backlog is a forecast and not a promise.
Stick with the authoritative: