If I were Garcia, I'd accept this fight. He doesn't need to demand too high a paycheck for this. It is acceptable to be given the smaller slice of the purse. After all, that share is still high enough, as you've said, might be his largest ever. Not to mention that he is able to fight against a legend who is already slowing down with age. Although he is not in his best form himself, Manny is definitely a lot older than him and has been inactive for a while.
Garcia's market is lower compared to Spence and Crawford, so he should accept this fight, not because of the opportunity to earn but to beat Manny, as the moment it will happen, his value will also rise and next fight, he can demand his price.
I doubt that Pacquiao will accept this offer. He has a limited time remaining on his career so it's common that he will pick a better opponent with a good profit return for him. Spence and Crawford is a strong fighter and at the same time has a high viewers expectancy. I believe he will choose between them since he has nothing to prove for fighting not so hot fighter.
OT, I thought Mcgregor is the one who he will fight this year though. IRC.