Boy, there are a lot of questions, and so many of the answers depend on your individual needs, goals, worries, etc.
First a shameless plug. I'm putting on a free event tomorrow in Mill Creek that will be addressing these questions. http://biznik.com/events/what-to-expect-when-youre-expecting-business
As a general rule, I would say there are two major reasons to split business activities into different companies. One is liability, as Valarie mentioned. The other is for ownership. A partner/shareholder/member has a legal claim to property and profits from a business. If you have a partner involved in part of what you do, but not in another part, that's a very good reason to have two or three LLCs. Likewise, if you see yourself adding additional owners in the future, do you want them to have partial ownership over everything or just over certain activities?
A lesser issue is that you can choose how to tax an LLC - as an individual, partnership, C or S Corp. If you have multiple LLCs, you need multiple returns, but you have the flexibility for each to be taxed differently.
Federal, state, and city tax filings are required, often even if you have no income. Usually, no tax will be due, and they're easy to fill out, but you do need to keep up with it. Most filings can be annual, but you might be assigned a different frequency.
The property tax affadavit is required, and you'll be assessed based on the current fair value of the assets used in the business. Snohomish county (and, I believe all WA counties) have similar processes because state law only exempts personal property from property tax, not business property or real estate.
As Valarie said, you should sit down with professional advisors for help with this decision. I recommend speaking to both a lawyer and an accountant. Each is a specialist in their own area (liability and law vs accounting and tax).