I'm not a super tax expert, but I have stayed at a Holiday Inn before.
Roth conversions will show up on line 15b of a 1040. This means that Roth conversions contribute to your Total Income, your Adjusted Gross Income, and your Taxable Income.
Generally, yes, I'm fairly certain that the converted Roth money will be taxed at your marginal rate.
The amount will literally be added to your income (by this I think you probably are thinking of W-2 income). W-2 income goes on line 7 and gets combined with Roth conversions on line 15b at line 21.
Yes, if the Roth conversion (plus your other income, such as wages or capital gains) increases your AGI (technically MAGI) beyond the contribution limits for your filing status, it would prevent you from making a Roth contribution. Speaking more generally, the increased income that results from a Roth conversion affects all sorts of things that are based on income, such as child tax credits, ACA credits, college financial aid, the various education tax credits, the Earned Income Tax Credit, etc.
I personally would not convert at the 25% bracket unless you knew your tax bracket will never be lower and you absolutely need to convert in order to start your Roth pipeline funding because you need to retire, like yesterday. The generally recommended approach is to find some other way to fund your first five (really four) years, and then start converting after you've retired. Then you can usually Roth convert more money at a lower tax rate. That's what I am doing.
Good luck!