How exactly does murdering the diplomats avoid the need to address the diplomatic situation? We really can't just stop them from delivering the message, and they almost certainly have confirmation that we have already received it. And if we want to pull a whole "you must help us against these people who attacked you" thing then, well, that won't even slightly work. They will probably blame us for not providing security in our region. That is not exactly an alliance-killer but it does put us at a disadvantage. Mostly though, it creates a situation in which they have attacked our allies, and thus we are ourselves caught in exactly the same "We must take a firm stand against this atrocity" trap. We will be doubly imposed upon to give in to their demands as a sign of the new sense of unity that has been forged by the murder of diplomats by our shared enemy. Or do we expect "What do we care, they were your diplomats" to be a viable diplomatic strategy?
I am happy to maintain our sovereignty, these are our citizens and their fate is our business. We are in a technological alliance, not a social one. The sooner that we just front up and make that clear the sooner they will either stop with these requests or break off the relationship entirely. Or, if we really oppose Germany so much(and please don't forget that while the Nazi's did get up to some particular naughtiness, no side was innocent, Foreignia included.), just end the alliance and be done with it.
If you want to get clever with diplomacy, then you need to actually be clever about it. Throwing our allies into an enemy fleet is highly unlikely to look like an accident(especially after we have already been recalcitrant on this issue), and even if it does, it won't just magically make our problems go away. Also, murdering diplomats is the magic key that unlocks all sorts of bad things. You only want to do it if your whole plan is to serve up a giant finger in the most insulting gesture that can be mustered(Which you all notably refused to do when we had the chance to use the pride of the enemy fleet as a landing craft) or have plotted out every tiny little detail so precisely that it makes Machiavelli's image look like Marie Antoinette's. Note that planning at that level generalyl doesn't involve enemy fleets, it typically avoids anything remotely as hostile as an enemy nor remotely as large and involved as a fleet, and if we aren't involving an enemy fleet then we may as well just shoot them all lest we risk a real enemy fleet turning up at an inopportune moment...
And I would prefer not to involve any seal gods.