Mussolin behave more subtle would deprive Hitler of a big role model. It was Mussolini's success led Hitler firmly believe that he could violate everything but still get away.
Sure, but as i said it was not stupid and worked diplomatically before starting his war and without some serious political pod i don't see the UK really want to start a war for Ethiopia while he had not want to star a war for the Sudetenland.
I think the political/diplomatic objections are the strongest part of the argument you put forwards but I would also argue the strength of them actually comes from the awareness by the Italian Government that the British could do something along the lines that would be seen at Mers-el-Kebir even if they were not aware of the kind of thinking that would lead to Taranto.
Essentially the worry was that even if the Italian Navy hid in port British battleships could shell its units into scrap metal while even worse any such withdrawal from the high sea would abandon both the Italian Empire and the armies in Africa.
So what you would need is a really sneaky British Government that would artfully lure the Italians into some seeming show of aggression towards British interests that would allow the Admiralty at least a fig leaf claim that its actions passed the Caroline Test. Now normally it is a good thing that such British Governments generally reside only in the fever dreams of Perfidious Albion (the concept not the board member named for it) but for a writer looking for an ATL that heads off the Second World War such an unscrupulous regime would do nicely.
Don't use Mers-el-Kebir attack as an exact blueprint for any possible event, there were political issues that favored the surprise attack by the British and it's more probable that any direct attack on Taranto by the British Battleships will go against the italian coastal fortification and batteries, not considering the air forces...basically the entire point of the existence of the bases of Taranto, La Spezia and Trieste were to avoid an event like that.
And you also need someone idiot enough to being lured in a trap like that and frankly, except for that single moment during the Crisis in Abyssina everyone in the italian goverment was very keen to not do that...one just need to see the ROE for the italian pirate submarine during the Spanish civil war.
in OTL the attack was planned and practiced for pre war, they tried to collect the pre war pilots who were involved in this for the actual attack. The difficulties with the shallow depth were overcome by attaching a wire to the torpedo so it would not enter the water at a shallow angle. The attack was planned to be by two carriers but Eagle's fuel system was shaken up by Italian bombing. I also think the attack was launched from 170 miles rather than 130 miles From Taranto.
The Italians lost the Illustrious strike group before the attack because Radar directed fulmars could shoot down spotting aircraft prior to them spotting the fleet a greater number of carriers in the strike group would not greatly change the risk of detection.
Cheers Hipper
IRC the modification was used after Mers-el-Kebir and on torpedoes not in service in 1935...and in 1935 radar it's not really a thing