OP, I think that you are wise to try to determine what the potential issues could be at check out time for your group. It is better to know and have a plan to deal with such situations before you go on a trip together.
Regarding late check outs:
My understanding is that you can request a late check out from housekeeping but it depends upon many factors whether or not it will be granted. We have requested late check out a few times and been granted it occasionally, but not always. In our case they asked how long we needed before we would be able to leave. WE always try to be out by 11:00 am, or as close as is possible, as a consideration to both the housekeeping staff that are challenged to get rooms ready for new guests and to the incoming guests that are hopeful to get into their rooms as soon as possible. Larger rooms like suites would be much harder for them to give late checkout privileges to because it would take longer to get these rooms ready. I would still ask, but wouldn't be surprised if the request was denied.
Requesting a late check out the evening before is much more likely to result in getting a late check out approved. I would NOT wait until morning. Late check outs, when granted, are on a first come, first served basis, I believe. If the number that they have allocated are all gone you will be denied.
The policy of no fee until 1:00 pm, 50% room rack rate from 1:00 to 3:00 pm and full night at rack rate after 3:00 pm is fairly standard in the hotel industry, but may or may not apply at WDW. If you have requested and been denied a late check out, I would not be surprised if I received a charge for any amount up to a full additional night after the fact. Hotels often place a credit charge 'hold' on your card to cover extra expenses that they may need to charge post check out, such as for rooms damages or whatever. If you 'forget' and are still in the room and housekeeping 'discovers' you, it is possible that they would report the situation. I don't know if this would happen, but I wouldn't be surprised. I doubt that anybody here can tell you for sure what your experience will be, because many scenarios are possible.
If it were me, I would simply tell my friends that if they are late leaving the room and there is a subsequent charge (up to the full night rack rate cost for a suite), that THEY will be expected to cover the entire charge themselves. Then I would request a late checkout and proceed to be out myself by whatever time was required by housekeeping. As you said, you cannot control how other adults decide to behave. You can control how you behave yourself. You can discuss how different situations (like this one) will be handled together as a group in advance. Such planning will probably result in a more pleasant trip for everyone.
Good luck! i hope that you succeed and get your late check out!