There is another option which is being discussed in Norwegian media now - that the Russians have used the pipelines essentially for storage of excess gas (Seeing as the pipelines are basically, combined, a 5M cubic meter pressure vessel)

That would go a long way towards explaining how several ruptures happened in relatively short succession - pipes being loaded up with too much gas (after all, the more gas stored, the more money in the bank for later on!)

Once the first one burst, someone probably re-evaluated that idea, but letting off gas in sufficient quantities fast enough may not have been possible.

Of course, if this was indeed the case, I assume it will very soon be very evident - I can't imagine that the rupture caused by an external force will be indistinguishable from that of it rupturing due to overpressure.

Doesn't make any sense since pressure is measured on the German side as well.

Simultaneous ruptures could be caused by sabotaging software. See my other comment; the US did something similar to Russia in 1982.

These days, it wouldn't surprise me if it was script kiddies on Shodan.

...but that would require flow, right? (that is, closing of the flow too fast will basically lead to a shockwave rupturing the pipe).

In this case, the pipes were basically being used as pressure vessels (I believe it was quoted in the media that pressure in one of the pipes was 105bar/1,500psi or so before the leak sprung).

With nothing but static pressure, I have a hard time seeing how such a pipeline may be sabotaged in SW alone - doubly so as it, as far as I can tell from media reports, really is the proverbial dumb pipe - valves and thingamajigs at either end, but the pipeline itself is essentially just a pipe with no shutoff valves throughout. (Which, in itself I find hard to believe, so I probably had better dig a little bit more!)

Can one explosion at one place in the pipeline cause vibrations down the pipeline, possibly weakening another joint/seal further on?

Not sure. I know nothing about underwater pipelines :P

Does all the gas leaking to the surface cause boats to sink if they get too close from the change in density?

There have been three explosions in two different pipelines, North Stream 1 (the older one, which was in normal use until earlier this year) and North Stream 2 (the new one, which was pressurized but never delivered gas commercially).