I'm not that experienced - I have one book published - but I have been hanging around with industry professionals long enough to get a basic feel for how the business works.
My gut instinct, if this applied to my work, would be to thank Solstice for their interest, but tell them I am removing the mms from the market at this time. Nothing else said.
I'd look at my to-be-queried agent list, and my query itself. Weed out the former and refine the latter, then send out small batches of queries every month or couple of months. If the book is strong enough to get a legitimate offer from Solstice, it will catch other publishers' interest.
I would not start querying publishers until I'd gone through potential agents. This also means identifying those agents who might be amenable to advising on a contract when you do get an offer - that's how I found my current agent. Some agents won't work on contracts they didn't shop around from the beginning, so pay attention to their blogs and guidelines. Some agents who are closed to unsolicited queries are more approachable when an author has a contract offer worth the agent's interest.
If I had no agent interest (and I stopped querying for one book when I hit the 75-agents-and-no-dice stage), then I'd apply some more market research for publishers. I'd try to identify strong mid-range or big publishers open to unagented queries. That means they are stable, they have a good industry reputation, they have a fair number of authors selling well, and their management is sane. You'd be surprised how important that is.
This process takes time.
