Searching is a highly debated process, and a lot of patent professionals will argue that you shouldn't do a search, or that you have to. I know that's not your question, but it's an important piece of the answer.
The short answers to your questions are: any company can provide a searching service, maybe, and yes.
Ultimately, the entirety of the burden falls on the filer, or filing attorney.
The standards for searching are pretty straight-forward:
- You don't have to do a search.
- If you do a search and find something, you have to tell the patent office.
- If you submit too much, you can get in trouble for trying to hide relevant parts.
Anyone can do a search. I could do a search right now, just using some search engine. You don't have to be a patent professional to do that. People who know how to search often are professionals, but there's no requirement there.
Beyond that, the liability comes only through your contract with the provider. The USPTO is holding you accountable for what you give them, so if you fail to disclose something you find, that's on you, and not on the searcher. However, of course, if they're giving some sort of guarantee on their searches, you could then hold them liable for not pulling through.
To avoid that, yes, they could offer a contract that reduces their liability.