1. There are other issues that types have. Say, Ground having Terra Strike as its only special move, or Ghost's main physical move being Doom Scythe. In Light's case, it's the lack of a high-powered accurate special attack. That's generally okay since that happens with a lot of types, especially before D/P (Bug, Grass, Fighting, there's lots). Of course in D/P 80 became the new standard, but it's not necessarily something we have to follow. Though Signal Beam is slightly more prevalent in Topaz, Silver Wind remains the standard Bug special move among other type disparities. I'd venture to say that Dolphure doesn't require a light-type move. Light fails to hit hard what resists Water (in particular Grass) and so even with a power boost to Light Wave Ice Beam would still be the superior option. Ice is just that good in canon games.

2. Yes, it might be too good. HM-wise I believe Flash has been relatively limited but from what I feel Flash is going to be the only really prevalent Light attacking move (Smite is good but its coverage is iffy unless you run Fire or possibly Psychic). In that case Flash is similar to Earthquake. But that's what the demo is for. Delay it until mid-game is entirely possible.

3. There are lots of pointless moves (competitively), even in canon. Frenzy Plant? etc. I would possibly enjoy seeing a more creative variation though, so feel free to suggest.

4. It being overpowered against computer opponents isn't very tangible, when you consider how simply overleveling, RBY Fire Punch Abra, Sleep, etc. breaks the game really easily. Is this really that big an issue?