Putting that aside, you are making a lot of assumptions as to the motives behind the decision here. It is within the realm of possibility that you might be right. But the only "evidence" presented in this discussion by any of the opposition to the decision has been nothing but conceit and disdain of Protestants. I don't find that convincing.
If somebody would like to present an argument that this particular case is a bad way to pursue ecumenical action because of particular criteria other than automatic contempt for all things Protestant, I'd be quite willing to listen and may well agree.
However, it started with nothing more than hysterical screaming about blasphemy and heresy. Once that toned done a little bit, we still have nothing more than "we're right and Protestants are wrong." While I agree with that last point as a general statement on theology, it does not follow that each and every individual action taken by a Protestant must therefore be morally incorrect.
But let's look at what you thought this was about; namely the "hysterical screaming about blasphemy and heresy," you claim to have "toned down."
First, heresy. Beyond the heresy of the Methodists in their denial of the Real Presence and other issues, we have the active heresy of Indifferentism that I pointed out and that you responded sarcastically by your own admission. Religious Indifferentism is a heresy, and where allowed to fester in the Modern Church has cost the Church dearly in terms of lost souls.
Now, how was this incident blasphemous? According to the Catholic Encyclopedia, " Blasphemy (Greek blaptein, "to injure", and pheme, "reputation") signifies etymologically gross irreverence towards any person or thing worthy of exalted esteem."
Probably not to the Methodists, who likely performed what they considered right worship, even though they are objectively incorrect on this. Nor, thinking a Catholic Cathedral as anything other than a venue for an event, did they objectively commit irreverence toward the Cathedral, especially since, again, they deny the Real Presence of Our Lord, present in the Cathedral.
Did anyone? Here I would say those that treated a worthy thing in an irreverent manner were those in the Diocese that allowed this service to take place. They know that it is the seat of the Bishop, and ought to know that our Lord is truly present in the Most Holy Eucharist there. Further, as the main Church for the diocese, it is of higher esteem even than a typical parish.
In truth, however, blasphemy does not apply because it is primarily a sin of the tongue. What was permitted falls under the sin of Sacrilege, which is "the violation or injurious treatment" of a sacred object (again referring to the Catholic Encyclopedia). Among the violations of a local space such as a Cathedral is the following:
the doing of certain things (whether sins or not), which, either by their own nature or by special provision of law, are particularly incompatible with the demeanour to be maintained in such a place. Such would be for instance turning the church into a stable or a market, using it as a banquet hall, or holding court there indiscriminately for the settlement of purely secular affairs.www.newadvent.org