Not sure if this is a silly question but wanted to make sure I dot my i's.
Are there any particular legal aspects that need to be considered or finalized before launching the stealth (sign-up) landing page? For example, trademarks, corporate identification, etc.?
Maybe... but if you're worried about that it's more of a sign in most cases that you're not focused on finding a real problem that has a solution with a meaningful market in my opinion.
My experience is that branding elements such as names, taglines, logos, etc - are not why people select a brand. At the point you're competing against other companies based on branding you've found a market, and know who the audience is for that market.
If you're really starting a startup, then it's unlikely you know who the market really is, or for that matter what model will really work for them and you. Focus on finding a real problem, then finding a solution that meets the needs of the market. After you have a market, then focus on branding.
Just pick a short, easy to spell domain, that does not conflict with the current solution your building. If the solution changes, just find a new name. It's not a big deal.
Trademarks and corporate ID are not a huge concern (in my non-legal experience): make sure you get the .com, of course.
Some countries have laws on how you handle customer data: if you are gathering data about users on the website, you may want to look into those.