Let me present a few arguments here, because I think your concern is totally valid and ought to be addressed.
Group A: Why Potential Imbalance Will Have a Less Negative Impact Than You Think
1: The game is much shorter, and any bad experience can be immediately overwritten with another one. Unlike a 75pt game, LPGs can be set up, go bad, be completely changed, and be re set up in a matter of minutes. This reduces the impact of NPEs considerably, since it will be rare for a player to be able to only play one or two games in an evening (and therefore often in a week).
2. The arms race that your goreshade scenario creates is what drives a dynamic meta - if Lamentation is dominating a few games, then people can bring casters that rely less on spells. If guns are ruling the roost, people can bring casters that are highly resistant to them. And if Butcher3 is running amok, people can bring things that stymie his approach. And this can all happen over a couple of weeks by the sheer volume of games that can be played!
3. This is not meant to be a 1:1 replacement of 75 pt games. The only thing I think should remain consistent is the list building, meaning no additional restrictions because it's an LPG. Terrain and scenario will need some adjustment, for sure. Bottom line though, balance being off matters less in a more casual format, and this becomes more true the more games you can play in a night.
Group B: How to Help with Existing Balance Issues
1. Terrain - setting up 8 (or even 8-10) pieces in the center 24x24 zone of the board with plenty of LoS blocking matters quite a bit and takes the edge off certain super solos. There also needs to be some sort of instant death killbox, we've had it work well at 12" from all sides, but less might also be fine.
2. Scenario - this one needs more testing, bit with 2019 objectives giving some sweet abilities I think they could go a long way to help out certain specific issues. Two objectives per side and 2 flags might be something we try, but the current SR packet is definitely not great for LPGs. But again, this isn't meant for tournaments.
3. Tournaments - I know I just said it isn't meant for them, but I mean for a standard SR. I think you could do some very fun tournaments where your list evolves as the day goes on. The small size and low time should be what is leveraged in creating a new format.
I think balance concerns are legitimate, but I also think they won't be as bad as feared. A single game won't be balanced, but I guarantee Goreshade1 won't stay at the top for long.
Support heavy faction fun issues: I think in the case of protectorate you are thinking too small (literally), because basically every caster can take choir and a Judicator, which we have had happen locally. It's actually still fun on both sides because it presents a unique challenge to both players.
It definitely requires out of the box thinking, and I won't deny that trolls especially can feel that support bloat constriction harshly - but I also think that dismissing it out of hand because of that is a mistake. As a skorne player, beast handlers take up 5/10 potential army points, and that forces me to decide if it's worth it or not to take them. Interestingly, the entire anatomy of the game shifts, and that impacts the value of support. At 75 pts, the amount of models the Krielstone, Choir, and Beast Handlers can affect is large enough to always make it worth it. At 10-20 pts, maybe not? It's worth a try, I think. And hey - if you can get 3-4 games in when you could normally only get 1, data gathering goes much more quickly!
Edit: also, I am going to create another thread for this since it isn't really relevant to the OP, please respond there instead.