I'm also a government employee (DoD GS 13) with my TSP. Did you know that during the Clinton admin that he authorized the government to 'borrow' money from the TSP? Not that I think we will ever lose the TSP, but it wouldn't take an act of congress. Or if a RIF (reduction in force) occurs, which does every 10 years or so in DoD. Or the BRAC (which is currently underway) occurs and your facility is closed and you have to move 1/2 around the country to keep a job.
As for the Roth vs 529, here are some more reasons:
1) Investment choices. Maybe you have no desire to take advantage of stocks, options, commodities, etc, right now. But you will soon. 529 plans limit you quite a bit.
2) If you over fund a 529 plan you either have to a) pass it down to the next generation (which isn't a bad thing in my mind, but to some it would be) or b) take the tax AND 10% penalty.
3) Flexabilty of use for the Roth. You can always use the contributions of a Roth for anything.
4) Your kids can always work or borrow money to pay for college. You'll never be able to borrow money for retirement. And if you aren't extremely lucky, your kids won't fund your retirement.
There are many things you should be able to do also, like possibly lowering your tax braket to 15% every 2 or 3 years. By realizing your capital gains in an efficient way you maybe able to have zero cap gains one year while maxing all your deductions (pay your prop taxes is such a way as to land two winter tax bills the same year, pay 13 mortgage payments that year too, give all your charitable donations for a few years in one year) as well as give appreciated stocks as charitable contributions one year (you don't pay taxes on the gains, but you get to deduct the whole ammount), max your 401K contributions. All that could land you just inside the 15% tax braket (I do it every other year) and then your Roth savins is only costing you 15% rather then the 25%. Regardless, my point is that you and I are roughly in the same boat, I can't see a reason to ever not fully fund a Roth yet still fund a 529.