Regarding your first question:
The traditional Jewish approach explains that the ones "standing" refer to angels, who are described as having legs but no ability to sit.
For example, the classic commentator Rashi explains:
העומדים. שרפים ומלאכי השרת שאין להם ישיבה
"The ones that stand: this refers to the Seraphim and the ministering angels who don't posess the ability to sit."
This fits with the vision Ezekiel had, where the angels are described as:
וְרַגְלֵיהֶ֖ם רֶ֣גֶל יְשָׁרָ֑ה וְכַ֣ף רַגְלֵיהֶ֗ם כְּכַף֙ רֶ֣גֶל
עֵ֔גֶל וְנֹ֣צְצִ֔ים כְּעֵ֖ין נְחֹ֥שֶׁת קָלָֽל׃ the legs of each were
[fused into] a single rigid leg, and the feet of each were like a
single calf’s hoof; and their sparkle was like the luster of burnished
The "single rigid leg" lacks the ability to bend, preventing the angels from sitting.
The angel of G-d is announcing to Joshua that if he repents, he will still merit reaching the "world to come" (which is the place associated with angels).