First thing is with the new way OOTO creates draft classes I would not encourage you to use feeders. That comes from a guy that used and liked feeders from the time they were added to OOTP. In v21 OOTP changed the way it created draft classes adding schools, stats, classes that develop over 4 -8 years (HS-College including junior college), etc. This IMHO negated the need to have feeders and the extra size of save files and places for something to go wrong. Larger file means more places for a save file to get borked. But to be fair I never had an issue with a save becoming corrupt. Nor did I have issues with my feeders once setup.
The thing for me discontinuing use of feeders starting with v21 is my save file is now literally half what it was before. I also see realistic dominant HS and College players in OOTP's created draft classes that are not likely found in feeders. Add to that I don't have to tweak league totals or modifiers each season to continue to produce the stats I want.
I do understand some still like feeders as you can interact and watch them develop your upcoming players in real time. That includes having news articles, awards, championships, etc. that you don't get with a created class. So if one wants to go that road it can be rewarding. If you want feeders because of the bold part, fair enough

. For me feeders were there to create draft players with stats, nothing more. So I don't miss them.
Having said all of that with regard to your #4 point. I think you would have to read that entire thread. Note the first post was last edited\updated July 2011. The last post in the thread is from November 2014. Almost 7 years ago. Feeders at least saw many tweaks in the following versions that may have finetuned some issues brought up in that thread. (and no I did not take the time to reread it from beginning to end today

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The biggest things for me with feeders was getting the players that filled the rosters in the original created league reapportioned. IE in my experience the league might be way to top heavy in P's or batters. So draft classes might come out 90% P's 10% B's (might be slight exaggeration but it's been a long time). I know I mass selected some players and moved them to even out the 4 class years for college and HS. This is something that might have been "fixed" in later versions and no longer needed?
1. when? If you've already started your regular MLB (pro?) season than I think it's too late. You want your league to have graduation before your draft so it can populate the draft pool. I'll tie this is with the schedule question.
2. Schedule? I set mine to ~54 games and had them end is late May so there would never be a conflict with my June MLB draft. I didn't want player still playing yet also eligible for the draft. It caused issues at first but may have been one of those things tweaked in later versions. So IIRC I would start my feeders around April 1, go with ~54 games, and make sure it ended by May 31. If the game created a schedule with those parameters that went into June I would change the start date of the season to late March and recreate the schedule. Once you get this right it should work every year, IIRC.
3. settings? Make sure you get the ages for both creation and max age per league right. But most of that is talked about in the old thread you linked. Figure out what stat output you want, set your league totals accordingly, and autocalc your league total modifiers before each season.
Do not expect real college or hs stats as far as dominant players like Joe Mauer who K'd like 2 times over a 4 year HS career IRL. Don't expect guys that dominate batting ~.500 and play like a man among boys. What you get is a league where the best players should show up over a 4 year career and you can look at them in the context of the league. IMHO feeders got a bad rap from many who didn't take them for what they were intended to be. IE a draft pool creator. Instead they wanted and complained they weren't a realistic college or high school baseball simulator.
Don't get me wrong it is s cool feature. Just one I personally don't need anymore for the way I play. If you go in knowing what to expect it does add depth and fun.
I hope you find something helpful in this post.