Weapon and armour proficiencies are not features of the first level of a class. They are an inherent property of the class which you get if you have any levels in it. Similarly the spontaneous spellcasting, domain spells, etc are not first level cleric class features; they are properties of the cleric's overall spellcasting ability, which it is explicitly noted the cleric would get as usual.
For the "the benefits gained at that level for the standard class", you should be reading the benefits listed on the class's advancement table for that level. If it's not listed on that table, it is not an ability which is dependent on a particular level of the class, and it's not affected by a substitution level unless explicitly called out. For the cleric, the first level benefits are simply:
- BAB +0
- Fort +2
- Ref +0
- Will +2
- Turn/Rebuke Undead
- some spells per day
Apart from spells per day, which is explicitly kept as if taking a normal cleric level, you just replace those qualities with the ones listed for the substitution level - in the cleric case, functionally just replacing Turn/Rebuke Undead with Smite Giants.
For the character mentioned in your note, they would get normal barbarian weapon proficiencies because those are a general feature of the barbarian class, not a benefit specifically granted at or by any particular level.
A beast master has one animal companion until 4th, then two animal companions until 7th, and then three animal companions until 10th. A 10th-level beastmaster has four animal companions.
This is not affected by how you enter the prestige class.
So, for your example of a 5th-level ranger (who has the animal companion of a 2nd-level druid), when he becomes a 1st-level beastmaster, he gains four effective levels in druid, so his (already-existing) animal companion becomes as strong as the animal companion of a 6th-level druid.
At the ranger/beastmaster’s 4th beastmaster level, this animal companion is now as strong as that of a 9th-level druid. He additionally gains a second animal companion, at his beastmaster level — 3 (read: 1st). In total he has one animal companion that is as strong as a 9th-level druid’s, and a second animal companion that is as strong as a 1st-level druid’s.
In the end, as a 5th-level ranger/10th-level beastmaster, he has four animal companions:
as strong as the animal companion of a 15th-level druid.
as strong as the animal companion of a 7th-level druid.
as strong as the animal companion of a 4th-level druid.
as strong as the animal companion of a 1st-level druid.
If the beastmaster were not a ranger, but rather a barbarian, he would be exactly the same except that his first animal companions would be as strong as the animal companion of a 13th-level druid, not 15th.
(As you might imagine, the actual usefulness of those low-level animal companions is seriously limited.)
Best Answer
The "any" is used to denote a choice, rather than inclusion. That is, your second reading is correct - it's any one level.
This is not necessarily clear from the sentence in the Elven Generalist Wizard description on its own, but it does become so when viewed in context with the rest of the system. Notice, for example, that this is the same language used by the Wizard class itself:
Here, the "spell level or levels" part makes it clear that the total number of spells gained is two. (Side note: This is generally not contested, see for example that the entirety of this question takes it for granted.) The wording in Races of the Wild is the same, only the lack of pluralization hides the context. Nevertheless, the context exists, and can lead us to the correct reading.
It also happens that this is the far more balanced result.