1. A Roth 401(k) contribution doesn't get recorded anywhere specific. If you made more than the legal limit of all 401(k) contributions of Roth and traditional combined ($19,500 for 2020, $26,500 if over age 50) then the excess will have to be declared and removed or penalized. I expect Turbotax picks that up automatically from box 12 but there aren't any further questions for you to be asked.
2. Under Federal tax law, DP benefits are not tax-free. I am not knowledgable as to the full history of DP benefits, but since currently same-sex marriage is legal, there is no fairness argument to say that DP benefits should be tax-free like spousal benefits. California might treat DP benefits differently and that would show up as a difference between your box 1 and box 17 taxable income.
Under Federal law, anything of value that the employer provides to the employee in return for their services is considered part of their taxable income, unless it falls under a protected category of tax-free fringe benefits. Health insurance is one of these benefits, provided the employer follows certain rules. The employee can pay their share of the premiums pre-tax -- and the employer does not have to include the value of employer-paid premium in the employee's wages -- for any plan that covers the employee, legal spouse, or dependents. If the employer offers coverage to any person who is not a spouse or dependent (including a RDP), the employee share of the premium is deducted after tax, and the employer-paid share of the premium must be added to the employee's wages. There was formerly a fairness argument about benefits for same-sex partners, but that was superseded by the legalization of same-sex marriage. Because you could marry your partner (regardless of sex, gender or orientation) there is no fairness argument to extend special tax treatment to benefits for unmarried people.
So yes, if you were married, the employer share of the premiums would not be added to your GF's Federal (box 1) taxable income.
3. That sounds about right.
4. The imputed income should have automatically been included in the box 1, box 3 and box 3 taxable wages by the employer and is subject to federal income tax, social security tax and medicare tax. (Whether it should be included in box 17 state wages would depend on state law.)
5. As an adult not related by blood or marriage, you can't be claimed as a dependent if you have more than $4200 (for 2019) of taxable income, regardless of the amount of support provided by someone else or who you live with.
