I also have no idea how to get it... your explanation makes no sense to me xD I'm sorry.
Since you've seen the Developer Guide, R28 is complete everything that's written in R29... and a little bit more, actually.
Visit all types of locations (forest, river, mine, ruins, catacombs, forest of illusions, house at the end of the forest of illusions)
Look at the Old Black Mecha Robot.
Bring home all types of food and feed Imouto each type of food once (Monster Meat, Fish, Vegetables, Monster Mushrooms, Monster Desserts, one spicy dish).
Feed Imouto milk and Rubrium Potion.
Give Imouto a good grope while she plays video games. To do this, you must purchase the Video Game Console from the Weird Knight.
After completing all of these steps, approach the knight every in-game day and choose the second option (That same word "R29").
After receiving the T-shirt, you'll have to wait at least one in-game day for the Knight to give you the DLC.
Afterward, the Knight will disappear, leaving only the letter in his place.
Now, about some other useful things you can do in RPGs, but not listed in R29.
Defeat the Cat Maid Swordswoman and learn her sword skills.
Rescue the Ninja Girl by feeding her food in the catacombs under the tombstone, and as a reward, learn ninja skills.
And now about the "RPG and SLG Connection."
Most likely, you need to catch Imouto playing video games at night.