With regards the Apocalyptic Methane Feedback along with what is described in the link you have just posted I fail to see how those are unlikely in the sense that Methane is already a record high in the Arctic.
The current levels of methane in the atmosphere are not necessarily a concern - the half life is fairly short (8-12 years?) and they could fall quickly if the human contribution was eliminated. While it seems very likely more methane is going to be released, provided it isn't released in such large quantities it overwhelms the breakdown mechanism in the atmosphere, it is a long term chronic problem (over thousands of years) rather than an truly catastrophic scenario.
It is of course possible (in the absence of sufficient scientific information) to speculate end Permian scenarios based on truly abrupt release and self sustaining positive feedback, but just because it's a theoretical possibility certainly doesn't make the two scenarios equally likely.
For me, I take the view that whatever happened in the end Permian was by definition a 1 in quarter billion year event (if not less frequent than that). That represents something truly exceptional and remarkably unlikely in terms of the earth system (and even so - we know the earth and life on earth both made it through in the end). Even if you include things like the PETM - the frequency of true mass extinctions on earth still isn't that high - they are all very exceptional events (at which point it ought to be concerning that our species is causing one just by acting as a super predator!).
I think the threshold of our vulnerability to change is low enough that it's a little irrational to be too concerned by the prospect of a PETM or end Permian scenario vs much more likely ones (that are still really rather serious for us).
Also something which the Arctic Ice Blog tends toe emphasize quite frequently is the whole idea of a mass Methane Expulsion from the Arctic in the sense that once the Arctic Ice is gone (let's say it happens this year) then the heat travels down to the hydrates, causes destabilisation and finally causing the hydrates to degass resulting in a large amount of methane heading into the atmosphere and causing the scenario that is proposed by the link to occur.
In my opinion the biggest concern is the reservoirs of methane as free gas thought to be trapped under the subsea permafrost (or is that permaslush now?). If I remember right, Shakova estimated gigatonnes of methane free gas to be available for release over decadal timescales (Shakhova and Semiletov tend to be very conservative, incidentally). A large earthquake, or submarine landslide, or gas under pressure rupturing bigger pathways to escape all present theoretical concerns here - but are
highly speculative.
Even if gigatonnes of free gas was vented into the system, I don't think it's a given that clathrates join in to provide truly unstoppable feedback - it still takes energy to melt them and time for heat to transport down to them. A one off abrupt shock from such release would be - well, very serious, like the change in Arctic albedo that's already underway. Not necessarily much worse?
I do think methane research should be a
much higher priority as even the chronic effects of significantly increased methane release are serious long term. Shakova and Semiletov will correctly caution that they don't know if they found new sites (or if the process has always been happening). I know in the deeper deposits off Svalbard it was determined recently that at least some (if not most or even all) of the methane venting was a process that has been running for a long time - longer than we've been adding carbon dioxide into the atmosphere potentially.
It is that sort of answer that we really need from sites like the massive emission sites discovered several years ago in ESAS (the geography of ESAS gives cause for concern - shallow water plus warmer river run off and removal of ice cover).
It doesn't take such an extreme scenario to cause issues though - if you look at Toba, the Younger Dryas - even relatively trivial climatic shocks such as 1816 (the year without a summer).
I am not saying it will happen, but with the amounts of methane already at record amounts and the dismal state of the Arctic it could be a distinct possibility.
Definitely maybe?
But I find it easier to focus on what we do know - and complain about the lack of urgent research into key areas that we don't.
Don't forget, there are always low probability threats with serious consequences - whether manmade (nuclear warfare, malthusian collapse) or universal (large rocks from space, pandemics, etc etc). Whether we perceive it or not, we have lived with existential threats for a long time now and the probability of the threat is key in considering how much time and effort it is worth?