Straight-up withdrawing from a Roth 401K is not the same thing as withdrawing from a Roth IRA. It is always pro-rated between basis and earnings.
As far as in-service rollovers, the plan has to allow it. Doesn't matter that the IRS allows this, which it obviously does or this thread wouldn't exist - only if your employer's plan allows for in-service rollovers.
Let's break down what you might be able to do with regards to the 401K - maybe that will help clear things up. Simpler, since you get no match from your employer, and you've got a Roth option.
1. You can contribute up to 18K per year in salary deferrals. Less if your plan limits you. This 18K can split between your Traditional 401K and Roth 401K however you see fit.
2. If your plan allows for after-tax contributions, you can bring your total contribution up to 53K this year - so that's 35K more if you go with the full 18K between traditional and Roth, even more if you don't max out the traditional / roth bucket.
#2 is most useful only if your plan not only allows for these after-tax contributions, but also allows for in-service rollovers. Based on your recent conversation, your plan may not allow for either of these things. Or the person you talked to didn't know. The fact that there is no match makes me think this very well may not be a feature-rich 401K. All of this will be in your plan documents - so get a copy of those, and read them to know for sure.