OpenRA

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You really don't need to learn about the OpenRA mini yaml parsing logic for the things you seem to be interested in. OpenRA is a matured software project which means that such fundamental question have been solved in some way years ago and most likely are not related to any issues you experience while doing your first steps in yaml-modding. There certainly can be bugs, but these will more likely in such a case be caused by specific implementation details rather than core engine features, like the yaml parser is. Then the right thing to do would be to report the specific, reproducible issue on github, probably using the bug template and providing the in that template asked for information.

Generally, instead of getting lost in C# code or things google spits out, you should IMO start learning from the many available modded maps as a first step. You don't need to learn about yaml specification because OpenRA uses its own flavour. Simply following that standard (even if it may be controversial or weird) is the best you can do to make progress in modding. The trait docs, examples to learn from and getting in touch with people who you can ask specific questions for a specific problem that you can't solve on your own is all you need to get things done. Approaching a probably easy to solve problem from the "root" by reasoning about the evolution of a language specification is certainly the wrong way if you want to create a map mod.

Although it is not the complete answer: use tabs for intendation in yaml files. Some other questions you asked are interesting but not relevant for your goal and have already been debated and are either decided or are still unsolved because of reasons that are noted in these discussions and again not helpful for you with your current ambitions and knowledge.

Some of this might sound harsh but I think it is helpful for you to progress.

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In addition to LawAndOrders's comment,
please don't open issues on GitHub until you have familiarized yourself sufficiently with how a working mod-code/script looks and how things come together in the end.

Most the time it's a really simple mistake which keeps you wondering for the longest time.

@SirCake: Thank you for adding your suggestions.

https://github.com/jimmiebergmann/mini-yaml has nothing to do with OpenRA. YAML actually forbids tabs nowadays. MiniYAML is something very OpenRA specific. It also has very few to do with object-oriented programming. It is more something like .ini, .xml, .json or .toml which stores data. However, it supports nesting which makes it special.