Short Answer: Yes, a mid-year change to the safe harbor generally requires 30 days notice, and accrues through that date. In addition to the amendment (which would be accomplished with the update onto the restated doc).
Long Answer: what provisions are unnecessary? Is the Safe Harbor NHCE only? Are you wanting to change the EACA and cross-testing as well?
He has a solok. The fact that it has provisions he might not use doesn't make it not a solo k . If the only two people eligible are him and his spouse, that's a owner only plan. Doesn't matter what the document is marketed as. Solo K is a marketing term, not a technical term.
How is everything max out already based on 25% of W-2? Most people don't have final W-2 wages until December? are they at 415 limits for the year?
Individual grouping (the cross testing) is super handy if the deposits don't occur exactly pro-rata. As long as testing passes they can be differing %.
I'm guessing the EACA was put in for the tax credit, even if its never used.
Hopefully the service requirement is 1 year, I've seen way too many employers think they will never hire someone and they do and that person is immediately eligible because that's how the plan is written. Having safe harbor to NHCE only also tends to help with this just in case it happens.