the old guy get a girl in the end this is a bad end?
the main char is shota?
the first stage have to do someting specific?
the options she gives - the right choice makes her happy influence the history?
From what I understood, the MC is an adult turned into a shota.
There is no story choices. In the 1st day (1日目), she's asking the MC how he want her to call him. But it doesn't matter, since she'll refuse perverted answers, which is most of them (at least in the beggining). However, in "アフター1" from the start menu (assuming you unlocked it), you'll get those choices again, though for some of them you'll need a higher relationship, or else she'll just insult the MC instead.
For penetration, it's only available in the last day (初夜) & free mode (フリーモード)
To do it, she must in a sexual position (accessible from brave menu), put the mouse to the appropriate place (for the vagina, the top right corner must say "膣"), hold left click for about 1.5 seconds and release, shove it in, and then left click again.
Hope I've answered your questions!