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Yes, raw linseed oil does not have any additives that would be harmful. So you can use it. And you can use a landscape liner to protect the wood further and keep dirt from washing out.

There are conflicting concerns about using stains, so be sure if you use one that it is labeled as safe for garden beds.

Untreated wood (which is best) will definitely rot away eventually. We don't recommend specific products, but I know there is some natural sealant on the market made from juniper oil that is supposed to be safe for vegetable gardening. Try looking online for this, or talk with your local garden center. It should be easy to find. It sounds like you don't need anything more for drainage.

Yes, it should decompose and enrich the soil.

https://www.gardeningknowhow.com/edible/vegetables/vgen/raised-bed-soil-depth.htm

You'll need to contact the company to find out whether there's any toxicity in this plastic. If not, the only problem I could see is that there isn't much porosity in plastic, so water may pool or not evaporate as well with a wood or stone structure. It may be a good idea to put holes in the material. I hope this works out for you.
You may enjoy this article: https://www.gardeningknowhow.com/edible/vegetables/vgen/raised-vegetable-gardens.htm

Steel will rust, unless it is galvanized or stainless. Those are common choices, though. Wood is best as long as it is untreated, but it will rot at some point. Natural elements, like rocks, are wonderful, as they do not degrade much over time. The same can be said for concrete.

Here are some articles that will help:

https://www.gardeningknowhow.com/special/urban/raised-beds-for-urban-settings-no-digging-required.htm

https://www.gardeningknowhow.com/edible/vegetables/vgen/raised-vegetable-gardens.htm

https://www.gardeningknowhow.com/special/starting

Far from it, unfortunately. Aluminum and zinc, at those concentrations will leach into the soil, once that coating protector fades. I took a look at the safety data for the coating, and found that in studies where aluminum was able to become part of the soil, the soil became toxic. Plants exhibited a build up of cadmium and zinc after extended periods of exposure.

This material should be replaced with a standard, zinc coated, steel. Unfortunately, the aluminum in the coating can create quite an imbalance.

Here is a study produced, where aluminum levels in the soil were studied:

https://pubmed.ncbi.nlm.nih.gov/31822411/

Here is an article that will help you to know what are good raised bed materials:

https://www.gardeningknowhow.com/edible/vegetables/vgen/raised-vegetable-gardens.htm