An oven/hob seems like a lot of work where a crock pot is nothing more then just turning it on and waiting. I'm not sure what country your in but in the USA there not expensive and worth the expense just for the sake of simplicity. I really don't know how to further advise you on this subject.
As far as whats safe to extract. I can't really say other then the usual tobacco, coffee, tea, mint, orange, lemon rind etc.. Other then that if it's an uncommon herb or product you want to extract do research on it first. You certainly don't want to poison yourself by mistake
As far as using alcohol for extraction we are talking about pure grain alcohol do not ever use rubbing alcohol. I linked to a good thread on this earlier.
Tobacco extraction using heated Ethanol
You should read it.
As far as sweetener most off the shelf sweeteners or powdered sweeteners are to weak to use and may contain by-products that may be unsafe to vape or may damage coils. It's best to purchase a sweetener from a vaping supply vendor.
From the questions your asking it seems you want to go all natural with as much extraction and home processed ingredients as you can. That my friend is an area that I am not an expert in and my knowledge on this subject is limited to the previously mentioned common extractions..