1) Does every type of seed need stratification?
Plants have adapted to survive in different environments. It is pointless for species in cold areas to have seed germinate as soon as they get wet because that's generally autumn then seedlings need to survive cold winter. Many of those species have adapted so that seed will not grow unless it has had a period of cold and wet then warm which means that the seeds now germinate in spring.
Other cold climate species have adapted by not shedding seed until spring so seed can't germinate in winter.
All that is a way of saying usually only species from cold climates need stratification - and then not all of them.
Species from arid and desert environments also can't afford all seeds to germinate with the first rain because it might dry out after and all dead = waste of effort. many arid species have developed hard seed coats so only a few germinate after first rain. Other seeds sit on the ground until either heat or time wears down the seed coat so water can penetrate. this means that some seed germinates each time rain comes so the tree is hedging its bets and chances are some will grow in good conditions and survive = success.
As growers we overcome that dormancy by either scratching the seeds (scarification) or heat treating (pour boiling water over the seeds) before planting to get better germination rates.
Species from fire prone areas have adapted to fit in with fire. Some retain seeds in hard cones until a fire goes through and triggers seed release - seed capsules are heated to release seeds. Others have inhibitors that stop germination until the seeds are burnt - seeds need heat treatment before sowing. Others are sensitive to chemicals in smoke and don't germinate until they get those chemicals - seeds are smoked or watered with smoke treated water before sowing.
Some species have hard coats or chemical inhibitors so seeds don't germinate unless they have had acid treatment as in passing through a bird or animal stomach. Usually species that have seeds in juicy fruit. Good strategy because the seeds are deposited with a neat package of fertilizer! These seeds germinate better after acid bath or leaching in running water for days/weeks depending on the inhibitor.
Other species have no inhibitors and seed grows as soon as it gets wet and warm enough. Those species are from milder climates or allow for huge death rates by producing huge numbers of seeds.
Many different strategies that plants have used to cope with the environment they have adapted to. As growers we need to know the strategies or make intelligent guesses at what germination inhibitors there may be.
The good news is that seeds that don't need treatment are rarely harmed by treatment so species that don't really need stratification will still germinate even if we stratify the seed so it is rare to harm seed by treatment, even if it over complicates the process.
Searching online will usually reveal which strategy to use with each individual species.
as Always!!
