Can a spouse assist in business operations without being considered an employee?
I run a small business out of my home (single owner LLC). A few years after I got started, my wife began helping with billing and bookkeeping tasks, but she was never made an official employee. She often makes business purchases using a credit card that is in both the company's and her name. Receipts also show company/her name.
You have a bigger issue than whether your spouse is considered an employee -- it's whether she's considered a business partner. By giving her the authority to make purchasing decisions on behalf of your company and handle other financial matters, you may be considered to have invested her with the "apparent authority" to speak for your company.
In the event of a divorce, she may be deemed to be entitled to a good part of your business, given the work she has done and the support she has given to you. If that's not what you want to have happen, speak to your accountant and attorney to find ways to compensate her so that it's clear she's not a co-owner of your business.