Dwarves aren't cool anymore

[Metastachydium]

3) They don't (repeated for emphasis). Higher level species could have been included. Their omission is a choice by WotC rather than a restriction on what WotC could have done. They have done it before (3E). This is one area where Psyren with insist on the status quo while I will critique WotC's unforced omission.
4) They do. WotC made a decision to have the power curve of PCs be an irregular mess. They wanted big irregular spikes at 5th, 11th, 17th and that makes everything, including higher level species, harder to design around. Multiclassing runs into friction around those levels. A higher level species is like multiclassing into a monster class (Troll 3 / Cleric 2 as a 5th level character).

Yup. Have I mentioned that the Midgard Dwarf discussed to death earlier is perfectly competitive with an Artificer (a strong, but annoying to use class in 3.5) of the same ECL?

Again, I am approaching things is a less-5e centric fashion, and thinking back to both previous editions and other games. For example, at what level do +1 or +2 skill bonuses become trivialized, particularly in a standard mixed party? And then how much later for a +1 save bonus to become marginalized (never trivialized, I don't think).

That depends on the skill system of the game in general. A 5e race granting an extra proficiency means that proficiency scales throughout the character's entire progression, while a 3.5e race granting a +2 to a skill means that bonus becomes less and less meaningful as the character goes up.

Interestingly enough, that's not actually true. It depends on the skill and build in question. 3.5 skill modifiers go higher than 5e "special purpose ability check" modifiers because while 5e ones might sometimes scale better on their own and automatically, 3.5 has an impossible amount of ways to add just another +1-to-Big Number to skill checks – racial bonuses being a source that stacks with all other bonuses.