I agree with intelligence being difficult to increase other than one time bonuses but both charisma and manipulation are easily trainable.
Do a seduction sesssion with a willing but not broken girl then just spam caress twice, something else like kissing once and repeat, you are likely to roll a +1 charisma before the session is over. And some kissing increases too, though they are not that important.
Then just do suggest sex, anal (any type really that you can have success with but anal is the first button), politely, continue, cancel, repeat. On the screen with the successful charisma roll and two buttons continue and cancel there is a chance for +1 manipulation increase. The cycle can be repeated until her arousal goes too low, but if she really likes you it can even go up.
The cap for both is 100, I can have both charisma and manipulation at 100 in the first month completely legit way, without F5 refreshing or savescumming, if I keep refreshing to get a crit roll it's basically one ingame day. Might just as well cheat to 100 to save time.