I assume that each of you is under age 50 and have no W-2 income that would put you over the Social Security wage limit when combined with your income from self-employment.
Your understanding in A-3 is incorrect. With $100,000 of net profit each, the maximum employer contribution for each of you is $18,587. This is because for a self-employed individual, the maximum employer contribution is 20% of net earnings, not 25%, because the employer contribution itself reduces the amount to with the 25% base rate is applied. Net earnings are net profit minus the deductible portion of self-employment taxes. With a maximum elective deferral of $22,500, the maximum total contribution for each of you is $41,087.
Assuming that your business is a qualified joint venture and each of you is reporting 50% of the income and expenses on a Schedule C, each of you should be able to simply mark the Maximize box for an individual 401(k) contribution and see the calculation of the maximum permissible amounts on TurboTax's Keogh, SEP and SIMPLE Contribution Worksheet. TurboTax's display in step-by-step mode is confusing, so I would ignore it and just go by the worksheet.
