Countering spies:
Infiltrate spy ring
High stability
Free subjects
Also I agree that spymaster should give spy defense
I'd also suggest adding postivie stability modifier: global_spy_defence = 0.10, doubling the difference between -3 stability and +3 stability.
I suppose fabricate claims needs some minor nerfing to not completely circumvent the excellent CB system. Making it more difficult could be useful, thereby increasing the effect of stability and other counters.
I think cheaper pirates is more natural than more ships. Pirates never threatened regular navies, and a letter of marque wasn't expensive in any way. Privateering operations were more or less self-sustained economically. Also compare the price of pirates with that of a galley, a ship more or less comparable in size. Giving a letter of marque should be less expensive than buying the fleet yourself. Another advantage of cheap pirates is that spies have uses all over the price range. Even small countries can afford pirates and thus avoids piling up a huge pile of spies with no use.
Also, incite merchants might be more balanced if it's more difficult, so that it can be avoided with proper precautions. Combined with the positive stability modifier it would give high stability trading nations some stability in their trading as well.
Updated suggestions
Increase difficulty for sow discontent to 8.
Pirates should cost 7 ducats.
Fabricate claims should be more difficult, at 6.
Sabotage reputation should give 3 bb.
Incite merchant difficulty should be increased to 5.
add postivie stability: global_spy_defence = 0.10
add spymaster: global_spy_defence = 0.03
Looks good enough? I so I might try it in a mod and post it as a suggestion if it works out as intended.