RideMax won't lead you astray. I've done it many times.
In fact, I made a mistake once and tried to get an FP for Splash Mountain. I tried to soon so I got the ticket telling me to wait until such and such a time to get the FP. The thing is, I didn't even look at the FP until we tried to get in the FP line at Splash. The CM must have seen the dismay on my face because he let us on through anyway.
Yes, you can use a fastpass after the time posted on the ticket. Cm's only look at the date and the start time. We gather fastpasses in the morning, head back to our hotel in the afternoon and then use the fastpasses in the evening when we go back to the park. We've never had any issues with using them after the time stated so long as it is on the same day.
Not in DL. I've heard that for some really popular rides in WDW, such as Soarin' and Test Track, on busy days, CM's have been asked to enforce the Fastpass return times.