Every body is different.
u don't have to flick the thermometer to reset the temperature reading when you use an alcohol based thermometer (unlike a mercury thermometer where you have to flick and flick and flick flick flick that thermometer for the mercury to be reset so that you can make an accurate reading) a disadvantage is that the alcohol thermometer is slightly less acurate The biggest advantage is that alcohol is not nearly as toxic as mercury, so that if the thermometer breaks, you won't be poisoned.
If he wants to, why not? Everyone has their comfort for going to sleep. For me it is reading until I fall asleep. If it becomes a problem in the relationship then he may need to get rid of it.
Answer I think that perhaps you've already tried this and know the answer, but yes you will dry up, she won't and you will still have an ejackulation when the time comes.