1. My goal is one update per two months max. So far I've been able to keep up with that schedule, but yeah, sometimes life can get in the way, so there might be occasions in the future where I won't be able to reach that goal. I can assure you though that you will never have to wait half a year or even longer for an update. Unless I'm getting kidnapped by aliens or other incidents in that ball park ;P
2. I've already written a rough outline for a story continuation into a second season. If everything goes well, Summer will experience many more years of adventures together with the bunch. There is some dependence on the financial success unfortunately (at least covering the development costs), not gonna lie, but I'm mostly doing it for fun, putting heart and soul into the game and the characters. And if people remain interested of course, that's kinda important too.
3. I'm working hard to build a structure around the game that prevents the need for a replay on current saves. I'm still fairly new to this kind of game development as well as writing in general, so there might still be future updates where I can't guarantee that a replay up to a certain part in the story isn't required, but I'll do my best to avoid this of course. I'm sure that I'll be able to prevent this from happening as soon as I reach the end of season one. Until then, it's still a learning process for me.
Hope that helps. Happy to hear you're enjoying Summer's journey.