I haven't ever kept sparkling gouramis but
@Matt has and so may be more familiar with their behaviour.
However, on the basis of my current and former experience of other fish:
(1) the little white spot on the gill cover:
* male goldfish do get breeding tubercles on their gillplates when read to breed - but I'm not aware of others species getting these (a bit of internet searching might reveal the answer to this);
* it's possible that an isolated white spot can come and go (my lone neon green rasbora has had one on her mouth intermittently throughout the ~18 months I've had her), whereas its development into a fungus or into more white spots would indicate something requiring treatment;
* no harm at all in having whitespot treatment to hand, in case you do indeed need to medicate this - if so, time will be of the essence;
* keep monitoring ammonia, nitrite and nitrates to ensure that water quality remains optimum as per currently.
(2) sluggish, heightened breathing rate, buoyancy struggles and swimming difficulties:
* [assuming he is definitely a he and not a female full of eggs and being harassed into breeding by the other male]...
* nitrite can pose buoyancy difficulties - but your level is spot-on, so that theory can be eliminated in your case;
* some species do breathe faster if constipated - I particularly notice this in cardinal tetras (quick internet search concurs with this);
* try fasting for a day, then feeding pea (shell off, each half cut into tiny morsels) to relieve any intestinal blockages that might account for what you're witnessing;
* keep us posted after trying this, and we'll see how to advise further;
* have some medication for bacterial infection on standby, in the event of need to use it (eg eSHa 2000, Waterlife Myxazin).