Solo 401(k) vs. Company plan

I have multiple clients that recently became owners of a larger group. (ownership % is less than 1%). The new group has a 401(k) plan for its employees, but they are also being told that they can setup their own plan through their S-Corps rather than contributing to the company plan if they choose to do so. Their own S-Corp is the partner and receives 100% of their gross income. They just receive a K1 (guaranteed payments) not a W2 at year end.

One of the CPA’s my clients are working with suggested that they are considered and affiliated party even though the ownership % is low since the majority of their S-corp income will come from the same employer for services performed from one employer. Others are saying the ownership % is so minimal that it dose not matter. It seems like everyone interprets this IRS publication differently. https://www.irs.gov/pub/irs-tege/epchd704.pdf

Second issue – Their S-corp is also receiving partnership income from their old company, which is in the process of being liquidated over the next two years. All employees were terminated in June and plan assets of the old company plan are currently being distributed. S-corp ownership in this business is 9%. If they open a solo 401k through their own S-corp (100% owner) is their a risk this could fall under the affiliated service group and there is a 12 month waiting period to open a new plan? The S-corp income from the new company is substantially higher than any residual income from the old company.

I have been getting conflicting information from 401k providers and CPA’s and do not want to take the risk that rollover assets become taxable or that a plan becomes invalid. From a tax standpoint it would be more favorable to do a self-employed plan from the S-corp than to join the new companies plan. Any advice appreciated. Also any recommendation for attorneys/CPA’s that specialize in this?