Again the capturing Luffy bit makes zero sense since boa is closer since was with him watching over him in those 2 years
you mean capturing coby makes zero sense and aokiji is with him 2 years. You said it like that right?.
so, Aokiji never has any contact with marines for two years till he saved smoker. Even though BB guess Aokiji is rat from marines,he's not completely sure bcz to aokiji lost a leg during the battle. Even marines wants him to go undercover ,there is no need to get lose a leg like that. Also Aokiji might have said he will fight against pirates but not marines as since it is his moral code related thing or he might have destroyed some marine ships already.
or he might use coby and make devon turn into coby and inflitrate somewhere just to kill someone like garp, or sengoku (i think he is the one who has death flag since he didn't show much of hisdf ability) to get his df ability.
or he might go after vegapunk?. Oda has tons of ways to make it crazier than ever.
This will only possible if he captured coby. If coby is outside then the marines will definetly have the news about him and devon cannot make any assassination kind of thing.
Also you forgot coby. the hero of the rocky port incident. Marines have good opinion on him and that is why it is perfect guy to infliltrate to make assassination to get strong df.