How to decide about items? (+sample game)

http://www.dotabuff.com/matches/2586503467

As a non-carry player, I'm managing the basics like not feeding, feeling where I can farm and where I shouldn't, knowing there are combats where if I ult and fight I can win (instead of always retreating due to fear of dying), etc. etc. While I get a hold of this, more doubts keep coming up.

Today I've stumbled upon another world of questions and tough decisions, and it's adapting to opponents. In the game above I'm Luna, and that is my go-to build. Whether Manta Style is bought immediately or stays as Yasha for a while varies from game to game, as well as the order for Satanic and Monkey King Bar. But unless something obvious requires me to deviate, I go for these items.

In this case, I won my lane and could free farm uninterrupted. Around minute 26 my inventory was Black King Bar, Power Treads, Town Portal Scroll, Yasha, Helm of the Dominator and a freshly assembled Butterfly.

- Is it a good timing for Butterfly?
- Would you gather your team and mass push towers in this situation, try Roshan, or just continue farming?

It wasn't until much later in the game that I realized their Elder Titan could strip away all my armor during clashes.

- Should I have built Assault Cuirass instead of Butterfly?

The second time we killed Roshan I was six-slotted and said I couldn't take the Aegis. My friend on Invoker insisted that I needed it the most, but I didn't even consider dropping anything.

- Should I have followed his advice?

At minute 45 I got my Moon Shard and told my team I had finished farming. I told them I couldn't get any stronger and that Morphling would outcarry me if the game went on. I told them it was time to push to their ancient.

- Can Luna actually get stronger?
- Were there any additional factors to take into account other than me getting fully farmed?
- Would you say the time to push was actually earlier?

I lost the last two teamfights and realized I couldn't do anything anymore. I had to pop Black King Bar so their supports wouldn't melt me, but both Mirana and Morphling had Ethereal Blade. All they had to do was wait until Black King Bar ran out and burst me into pieces. Manta Style wasn't enough.

- Should I have bought Diffusal Blade?
- Both teams had megacreeps. Should we have tried to split push instead of going all mid?
- Anything else I should/shouldn't have done?

Thank you very much for reading, and for your answers :D