Prediction and validation with log regression

[adikii]

Hi,
I have 2 regressions I'm trying to evaluate, both are based on the same data, but the second regression has log transformation:

reg<-lm(AmountSpent~Age+Children+Catalogs+Gender+Married+Location+Salary,data=train.df)
summary(reg)
pred.train<-predict(reg)
library(forecast)
accuracy(pred.train,train.df$AmountSpent)
pred.valid<-predict(reg,newdata = valid.df)
accuracy(pred.valid,valid.df$AmountSpent)

reg.log<-lm(log(AmountSpent)~Age+Children+Catalogs+Gender+Married+Location+log(Salary),data=train.df)
summary(reg.log)
pred.train.log<-predict(reg.log)
accuracy(exp(pred.train),train.df$AmountSpent)
pred.valid.log<-predict(reg.log, newdata = valid.df)
accuracy(exp(pred.log),valid.df$AmountSpent)

The questions I have are regarding the second regression:

1. Is it okay to use log only on one (or some) independent variables, while the rest of the variables remains the same?
2. When I use accuracy(exp- it changes the value only for variables with log, or changes the value for all variables in the regression?
3. Do I need to use the exp function on both the prediction and validation or just the validation?

Thanks!!