Hi! Having sold a number of different products and services to companies ranging from one man operations to Fortune 500 companies I believe I can help you get some clarity, despite not being an expert on retreats.
First- I'm willing to bet you have more questions than you listed. Is that accurate?
Do you think it'd be easier to talk it through over a call?
If so, schedule a call, and I'd be happy to talk you through these questions and assist with your whole sales process, your funnel, and goals, as well as the tools and tactics needed. One key strategy is at the end of this answer.

We're missing some key info, including - What type of retreat you're trying to sell, what's the price point, who are the target companies, etc... which would definitely help.
Absent these answers, I'll do my best to address your questions.
- Design the sales funnel after you have a lot more information about your customer and their buying behavior. I’m happy to help with this over a call.
- Prequalifying depends heavily on a lot of different factors we don't have just yet, but here's some general sales insight. The two best pre-qualifiers are: A) can they afford your product/service and B) do they have both an actual and, more importantly, a perceived need for it?
- Should you cold call/email? YES! That’s an emphatic yes. You can and should cold call/email/follow/connect/DM your target customers!
-Who to contact will be something you learn quickly when you get some more info.
-As far as what companies look for in a retreat - I have guesses, but I’m sure you know that answer better than me. Better yet, let your customer answer that for you.

To wrap this up, I’d say there’s a key strategy that would benefit you greatly at this stage.

Do some Sales Dev. Find a number of companies (I’d suggest at least over 100) that you’d believe could be your customers and get the contact info for the key roles that you think would be Power Sponsors or Decision Makers- I can help you with this step if you’d like. The best next step would be to set up calls to get more information (not selling, yet!) and the second best would be to email them a survey/form(I can help you set this up if you need). The goal of this action would be to find your questions above plus get all the extra information that will help you sell. Sometimes just having a contributing voice in a yet-to-be-developed product or service is cool enough to the participant that they don’t need extra incentive. It’s typically best if you incentivize their participation with some type of value add. Examples a % discount off the service when it’s offered, pre-sale discount pricing, access to aggregated information that’s valuable to them, and sometimes even a starbucks or amazon gift card.

Please let me know if this helped. This sounds like a fun and interesting business. I’d love to help out.