So, for all those still having trouble... I just finished playing 15.3 from start to finish (full cookie jar, except for those locked by specific choices). PC Windows 10 and...
a) All previous saves deleted from %appdata%/roaming/renpy/SummertimeSaga, and from game folder.
b) Clean download and extract. (Any previous version deleted.)
c) No patch.
d) Played the 'cheats enabled' version. (Loads of cash, skipped a lot of mini games. F**k karaoke.)
e) Default character names for Debbie & Jenny (Not sure if this makes a difference, but I didn't want to mess with ANYTHING.)
For the choices, I used the 'Girlfriend' route for Erik, and chose Mia over Helen.
Made it all the way to the end of available content without a single cheat code. Never even unlocked the console.
Four minor glitchy things I noticed, which did not impede progress:
a) Diane's "Mouse is actually a shoelace" scene repeated.
b) "Bug infestation" for the garden wouldn't let me look at the bugs (no way to click on the garden), so I had to guess (actually remembered from previous play) which description to give the store clerk when buying bug spray.
c) In the scene where Jenny confronts the MC in the hallway about sleeping with Debbie, she ended by telling the MC not to forget about the props needed for the cam show... but the cam show arc hadn't actually started yet.
d) Similar to c, Diane reminded the MC about "doing the things we talked about" (i.e. picking up the package from Pink) before she'd actually asked him to do that.
In all four of the above cases, I just ignored things and kept playing. No actual problems arose from any of these glitches.
And that's it. Overall the game played great and it was A GREAT GAME. I cannot wait for more content. (sigh, I know, don't hold my breath. I wish I had money to support to this game, job interview soon, I hope...)
I think my fave scene is the painting class, with Mia, Judith and Ms Ross... mmm.... I dearly hope that there's a proper MFF threesome at some point.
Thanks to the creative team, and to all the f95ers who offer tips, hints and tech support. Much love.