Statues are supposed to be kind of a surprise attack. You're supposed to adapt your strategy so you don't get overwhelmed by statues. Btw, statues are only a problem in the first house. After that, you barely see them.
About gravestones, the strat I've seen everyone do, including myself, is to destroy them with puff shrooms in the early game, specially sun shrooms. Grave busters has a property, when it destroys a grave, this one won't spawn when called by a flag as some others will do. Also, gravestones placement is not random, there are spawners pre placed.
Necromancers are very easy to kill and bait, you just have to place puff shrooms or even sun shrooms in other lanes and wait til your real attackers kill them. You can even use iceberg lettuce to avoid them to do anything.
About the night boss, I still think it's pretty easy, the order of the attacks it's always the same. Personally I keep at least 1 or 2 grave busters in the conveyor so I can use them against the spells that spawn graves, and use the rest to remove th special gravestones. I think the plant selection is good, by having the 3 mushrooms, 2 wall plants and premium plant that affects the entire lawn (which I also keep at least 1 for emergencies).
You even have lawnmowers, so even if the early game is overwhelming, you can continue the fight. I'm currently watching captain flare's streams beating the entire game, and I've only seen 2 levels that are very bad, one being 18 and the other one, level 44. I think the rest of the game is fine, although some hard mode levels like 1-4 feel more like experiments rather than actual levels lol. (Btw, in the next patch I'm changing those 2 levels I mentioned)
I might increase the money production because you're right, it feels kinda slow, and the only way to get a lot of money is by playing whack a zombie, or maybe even playing endless since zombies drop more money there.
By last, you cannot unlock achievements before level 7, that would make some achievements trivial cuz you would go to level 1 to unlock them.
Thanks for playing and for the feedback, mate! I'll check some of the stuff you mention once I have time to work in the game :b