Score - why should player care about it ? I think majority are in for sexy stuff - of course taste differs for everyone therefore some might wish to end the game prematurely when certain deal don't really click with player. About MC motivation - she should do it for money obviously - but this money should also serve a purpose in the game, so that people want to earn more - not just for abitrary value at the end of the game.
Deal selection is a better system - it gives some control back to the player.
Tough to tell really - this seems like journey more important than destination type of game.
I think design itself will promote replayability. I take it will be impossible to get all deals in single playthrough - as well certain playstyles. Since relationship will be cash based - you will just need to create moneysink to make bigger payments more attractive to player (either more difficult or more humiliating)I think some sort of modular deals so that butt groping could progress from just in the wild, to public groping by merchant in city. And so on.
That approach also sounds good - as 400 coin extra for each future trip by allowing merchant to fondle her. In this case certain deals could also come at disadvantage - obviously skimpy armors would provide worse protection. Some sort of license system so that you can buy/equip X type of weapon only if you have certain license. Or maybe even clothing/armor license system in city where skimpiest one would be the cheapest(MC of course wouldn't immedietaly be able to wear them). Of course she could get harassed in the cities lowering her morale a little - which could be regained by spending cash on relax. Another money sink could be potions lasting one combat/city turn enhancing MC abilities either combat, or social. Potions of course could have side effects as well.