Well you leave little alternative other than doing nothing and watch your plants die.
Then again we haven't actually established the larvae are fungus gnats but your description certainly sounds like they are. And the larvae may be a symptom rather than the cause - dying plants - more dead matter - more food source for fungus gnats.
Any treatment is weighing the risk to benefit and likely outcome.
1) Do nothing. Risk - high or none depending on how you want to look at it. Outcome - Dead plants based on previous outcomes.
2) Repot. Drosera aliciae has a robust root system and are robust plants. You'd eliminate most of the larvae, pupae and eggs, a sand mulch will prevent future infestations. Risk, very little. At the beginning of summer I repotted my 6 year old colony of D. aliciae that was in serious decline. The peat moss was breaking down to the point of gaps forming between the plants. Some crowns had turned brown and apparently died. Flowers had been aborting for the past 3 years. One plant the roots snapped off. I trimmed off the skirts of dead leaves, threw out a couple of "dead" ones and repotted everything else include the rootless one in fresh media. Rootless plant resumed growth after about 6 weeks. Healthiest plants flowered and judging from the seed capsules (just harvested and drying as we speak) seed yield will be spectacular. "Dead" plants sent up 3-5 offshoots with more on the way. I have more growing points now than I began with. Risk - some to very little, outcome - very good.
3) Hydrogen peroxide. I've never heard of using this to treat fungus gnat larvae. I suspect concentrations high enough to kill larvae won't do the plants much good either. I've used stabilized H202 to treat the soil of sundews before with no deleterious effects. I suppose it depends on the stabilizer, but should easily flushed out media. Risk/outcome - unknown
4) Bti. As Jesse points out it may or may not be effective, depending on the strain of gnat involved. Risk - very little, outcome - excellent (if it works).
5) Insecticides. According to you - high risk. Based on experience with compounds generally acknowledged as "safe" (with cautious use), Pyrethrins, neem extracts, Orthene, Imidacloprid (Provado) are effective against the species/genus they are meant for with little deleterious effects to the plants. What usually cause problems are the inactive ingredients, binders and carriers. Products in aerosol cans are particularly suspect due to the nature of the propellants, binders and carriers. Outcome - depends on the insecticide, if it was properly applied and the target organisms.
6) Sand mulch. Guides that recommend this say to use as a preventative so how effective it is with an active infestation is unknown. However it seems to me if the adults cannot get to layer of peat moss to lay their eggs reinfestation would be less likely. Risk - little to none other than the plants might expire before the "egg" generation pupates. Outcome - questionable due to the active infestation, otherwise very good (gnat free).