
Backing at any point after acquiring 1000g is completely fine for the strength increase.
Last item is personal preference. Can do a different starting relic, build brawler's instead of sunder to have more defense late game, or a different item altogether.
More mana sustain, better clear, extra health, in exchange, less prots, slightly worse fights. When building ypu can probably replace either breastplate pr genji's as you do not need the mana as much.
Less health and prots for more cooldown and useful active.
When you have bluestoe and don't need the mana regen, you get more health, less prots and cooldown, but a better passive. Also allows a different last item as you are not as health hungry.