Everything because thats what would happen if Spain joined the Axis.
And i'm still basically reading this as 'Lets kill British military personnel! Why? Because'.
I propose a new thread.
How to Sink the Bisma...oh...wait
There basically is no point at all for Spain to allow German armed forces in, for a purpose not benefitting their own. If German forces enter the state of Spain, Spain will simply become an ally of the Allies, siding with the UK and not the opposite. Spain wanted simply to be left alone to rebuild the wrecked state after the bloody Civil War. Anyone frustrating this would be considered an enemy, Fascist, or no fascist alike. Sympathy for Fascism was one thing. Fighting alongside them a completely different one.
So basically the idea is flawed and based on too many what if's to be of any use. A more logical assumption would have been a less complex approach, involving much less politically depending issues, with weapons and weapon carriers that could be deployed without too many problems. Suibmarines were the most logical to be used, though in 1940 the Kkriegsmarine had only a few operational, which were also needed for other more urgent missions. Only after 1941, the BdU could boost this number in more serious quantity.
So let me get this straight - you are suggesting that the Luftwaffe attack HMS Rodney from airfields well within the range of Rodney's main guns? In what possible universe is that going to end well for the LW?
The reasons why spain didn't fight are known. The concept in the OP is to provide an alternate history where Rodney could be sunk. Spain joining the axis isn't ASB, they were allied and Franco really did consider it... he could change his mind looking for the glory of bringing gibraltar back to spain, or the axis could support a coup by the more radical elements in the falange who want to join the war or anything else that drags spain into the fighting; either way it's plausible turn from OTL
I am at work so I don't have my references at hand, but the Germans developed Air-to-Surface guided missiles (used to sink an Italian battleship and heavily damage a number of allied cruisers) and guided bombs.
These could have inflicted major damage on the Rodney's.