In fairness, my last post was a little pedantic (quick warning, this one will be too). I'm a bit more on board with this post of yours compared to your previous one because you're actually talking about potential real economic consequences of government debt (which I'll get to in a bit), but your language is still bound to some frame of reference where surplus always equals good and deficit always equals bad. Is a little bit of something necessarily bad (whether it's "not that bad" or "could be better") just because a lot of it can be, even if that little bit is just what the doctor ordered (sometimes literally)?
In your previous post you said the government's financial position could be better. If you're willing to get a little abstract, it can be argued that the word financial (which has a lot of baggage when compared to fiscal) doesn't really apply to the (consolidated) government sector. Last year we had a situation where the RBA, which creates money out of nothing as a matter of routine, sold nine billion dollars worth of bonds because the rules say it had to in order for the government to give it nine billion dollars which served no real operational purpose but made sure that uninformed people didn't get the idea that the RBA could somehow go broke. That's not finance, it's some bizarre meta-game. It's a software work-around because you can't find the source code. It's like a group of people went to great effort to chain up a glacier and stop it advancing during winter, but come the summer only a handful of them are willing to acknowledge that the damn thing melted and turned into a river.
Ok, enough ranting. I'm just going to go over your points in broad strokes it that's ok. First thing is that it's not just having the keys to the printing press that has implications, it's also being the only one who issues bonds in your currency (though the two obviously go hand in hand). There's a reason all these "sovereign" debt crises are coming from Europe whilst the US just keeps plugging on (voluntary shutdowns notwithstanding) and Japan keeps doing whatever it is it's doing. So there's (theoretically) no risk of default, no competition (unless you go to high quality corporate debt), plus the government can also buy it's own debt; obviously this has significant impact on the behaviour of the bond market.
As far as your interest rates scenario goes, I'm not sure I agree that the government will be "overwhelmed" by repayments because, once again, no risk of default = consistent demand for bonds = no risk of default. You're right that there are downsides to a ZIRP (upsides as well, but let's not get into that), but if we have a scenario where government debt levels are high enough to make them the priority of central bank's monetary policy (e.g. maintaining 0% for fear of a repayment crisis), there is modelling which shows that raising the interest rate stimulates the economy, because it basically forces the government to take an expansionary fiscal stance strong enough to counteract whatever drag there is from increased private debt repayments. Stimulated economy = increased revenue/decreased expenditure = declining public debt.
As you said, all these things have consequences, good in some ways, bad in others. The important thing is to focus on those. Imagine a theoretical Australia where the economy is going gangbusters, with full employment, low inequality stable private debt but rising inflation, where the budget is brought into surplus by the automatic stabilisers. In that scenario, people like me shouldn't be criticising the budget just because a large enough surplus destroys private savings and drags on demand or because of any pointless esoteric shit about the government not needing to finance spending in its own currency, because the real consequences are suitable for the macro environment.
Anyway, I think you overlooked the worst consequence of rising government debt: it gets the Liberals elected and gives them an excuse to start butchering our society and economy with austerity
On QE, as I understand it it's more of an asset swap; "new" money is created and used to buy debt (government and sometimes high quality private) from the private sector. Depending on the situation, it can have limited impact because the money is really only one step above what it bought in terms of liquidity, but if private sector finance is FUBAR it obviously has a stabilising effect. It also increases the price of the assets purchased/lowers the yield, but if we look at the situation in the US it seems like it's all a bit of a wash in terms of stimulus, probably because if the private sector was going to spend the money on productive investment it wouldn't have bought the financial assets in the first place. So yeah, you probably already knew all this, it's printing money with a "but".