Not sure on Russian internal sabotage. The old "cui bono?" principle. Short of anarchists (of which we haven't heard in Russia at this time), which Russian faction would have an interest into taking out serious infrastructure of their own in the middle of a shooting war that they are losing?
Not sure on false flag either. False flag ops would have involved large civilian targets with large casualties just like Russians did before (under Putin's guidance none the less) in preparation for Chechen war part 2. Also last target was way too far for Ukraine's short ballistic missiles Tochka-U or helicopters (assuming they'd even be able to penetrate that far into the Russian ADA).
Occam's razor principle would indicate Ukrainian SOF. Taking out strategic stuff while limiting any collateral damage to civilians, in order to hamstring the invasion forces. NOT bragging about, so not to give Putin a reason to convert his "special operation" into a full fledged war for the reasons you mentioned. Russian maintenance is well known (even before the current affair) as being lacking and/or corrupt, so you have plausible deniability
And they'd be able to blend in pretty easily, while the current "special operation" RoE limis some of the security measures Russians could take publicly to protect the targets.
That's third hand news involving two low reliability sources: a German tabloid and a Belorussian opposition web TV service. I don't think that any missiles recovered from the bottom of the Black Sea after at least more than a week would be able to be used safely in combat. On the other side, the recovery vessel Kommuna is there, trying to reach Moskva's resting place. That gives fuel to a lot of interesting questions about what are the Russians so desperate to recover right away. I don't buy the secret documents/military equipment as what's the rush? No one else can recover them at this time, not with Turkey blocking the straits for the duration of the conflict. Nukes, maybe?