Don't have any experience with Steam. In fact I don't even know what it is and why I would or wouldn't need it...
The folder structure part in fact is very simple. Every time my son and I manual save a replay, the default folder shows up (in our case ...\Documents\Maniaplanet\Replays). In this folder I made 3 new folders (\Canyon, \Stadium, \Valley), we open the folder of the game we are playing and then save the replay. It's very simple, but it works perfect. As said, this was done because we had Canyon replays overwritten by Stadium replays when the trackname and the time where exactly the same. Ok, it almost never happens, but still, I didn't save the first replay for nothing, so I probably want to keep it. And when in "driving mood" I don't want to bother myself with this issue. Saving a replay has to be a (at most) 2-click action and off we go to the next attempt...
And when we have driven a race which needs some extra info, we just add this info to the default name, given by MP. For instance when I drive an official race and gain points, I always save it (at least if it's worth saving

) and add " - official" to the given name. Same goes for author medals which were hard to get. Since the time of the author medal is not shown in the track medallist by default, my son and I help eachother by saving the authormedal race, so the one who didn't get it yet, can see what the target is.
The autosaves are something I never look at and never use. In fact, I only recently found out they were there when I had some trouble (I thought) with wrong or missing best times shown in the upper left corner of the screen during the race. Someone told me it's because of changing your nickname. The autosave use your nickname as part of the filename, so when you change your nickname, autosaves from before are not recognized anymore and therefor useless if you don't know where to find them. If you do a lot of racing (and we do!), this folder can get pretty crowded. I cleaned up our autosaves at the beginning of May and right now there already 110Mb (almost 300 files) of data in that folder. You can turn this feature off (or check the option of "not recording replays"), but by default it saves every race you finish, which means a lot of data, which becomes more or less useless when you change your nickname. At least in our case, because we manually save replays we think are worth saving. However, as far as I can see these autosaves are used to display your best time in the upper left corner of the screen during the race. So when you change your nickname, the first time you drive a track with this new nickname there's no autosave of this track so nothing is shown in the upper left corner during the race. When you finish this track for the first time, the replay is saved and there is data to show in the next run. Until one of the last updates, your best time ever (assuming you didn't change your nickname for a while) always was shown in the upper left corner (ok, since it was implemented a few months ago). At this moment, this time gets messed up. When I for instance start Valley and open any track, in the first race my best time from the day before is shown in the upper left corner during race. It's not my fastest time ever, but my latest fastest time. I don't like it, but I can more or less live with it. However, when I finish this first race, in the result screen (and replay of this race) this race is shown as my fastest race. Still I can live with this, because it would be my fastest race of this session, which is correct. But when I click Improve (I guess that's what it says in the Egnlish version), in the upper left corner the fastest time of yesterday is shown, not the time I just drove. So in some way both times are incorrect now. The fastest time shown in the result screen is by far not my fastest time ever and the time shown during the race is ??? well, don't know what to do with this one. And when I have the guts to choose opponents form the result&replay screen after the race, all is resetted. Both the fastest time shown during the race (all data seems to be lost) as well as the fastest time in the result&replay screen. This goes for Canyon and Stadium too. I'll try turning off autosaves and clear this folder to see what happens... I just don't understand why 2 different times are used to display the fastest time, because in the main screen with the 15 (or 5 for the E-tracks) everything is correct. And when you want to choose opponents, the times shown are correct. I just don't get the purpose of this feature. What do you want to show: the best time ever? The best time of this session? The best time of the last few minutes? The best time with and/or without using the option to choose opponents?
Maybe I'm the only one having this problem. I don't mind if this would be the case, because then I would know that it's a local problem and not a bug...
Edit: The option to turn off replay-saving doesn't seem the funtion. I checked the option, so there shoudn't be any replays saved, but there are. Too bad...