После разговора с начальницей мы окончательно решили оставить старый план.

Because после always requires the genitive in Russian.

• после разговора = after the conversation
• Base form: разговор
• Genitive singular: разговора

This is a very common pattern:

• после работы = after work
• после встречи = after the meeting
• после ужина = after dinner

So после разговора is simply the normal case form after после.

Because с meaning with usually takes the instrumental case.

Here, с начальницей means with the female boss / with the manager.

• Base form: начальница
• Instrumental singular: начальницей

So:

• разговор с начальницей = a conversation with the boss

This is the same pattern as:

• с другом = with a friend
• с сестрой = with a sister
• с учительницей = with the female teacher

Because начальница is the feminine noun, while начальник is masculine.

• начальник = a male boss / manager
• начальница = a female boss / manager

Since the sentence refers to a female boss, Russian uses the feminine form.

In the instrumental singular:

• с начальником = with the male boss
• с начальницей = with the female boss

Here оставить means to keep, to leave as it is, or not to change.

So оставить старый план means something like:

• to keep the old plan
• to stick with the old plan
• to leave the old plan in place

It does not mean physically leaving a plan somewhere. In context, it means they decided not to replace it with a new one.

Because it is the direct object of оставить.

They decided to leave/keep what?
→ старый план

For inanimate masculine singular nouns, the accusative looks the same as the nominative:

• Nominative: старый план
• Accusative: старый план

So even though it is accusative here, the form does not change.

In Russian, решить is commonly followed by an infinitive to mean to decide to do something.

• решили оставить = decided to keep / leave
• literally: decided [to leave/keep]

This is a very normal structure:

• решили поехать = decided to go
• решили подождать = decided to wait
• решили изменить план = decided to change the plan

So решили оставить is just the standard Russian way to say decided to keep it.

Because the sentence talks about a completed decision and a single result-oriented action.

• решили is the past plural of решить (perfective)
  → they made the decision
• оставить is perfective
  → to leave/keep something as a completed choice

Perfective often focuses on the result. Here the idea is:

1. They had the conversation.
2. They came to a final decision.
3. They chose to keep the old plan.

If you used imperfective forms, the meaning would shift toward process, repetition, or incompleteness.

Окончательно means finally, definitively, or once and for all.

So:

• мы окончательно решили = we finally/definitively decided

It adds the idea that the decision is settled and no longer under discussion.

Compare:

• мы решили = we decided
• мы окончательно решили = we made a final decision

It is understood as part of разговора:

• после разговора с начальницей = after the conversation with the boss

So с начальницей describes what kind of conversation it was.

It does not mean we, together with the boss, decided...
If Russian wanted to say that more clearly, it would normally be phrased differently.

In this sentence, the natural reading is:

• first there was a conversation with the boss
• after that, we made the decision

Yes. Russian word order is relatively flexible, although different orders can shift emphasis.

The original sentence:

• После разговора с начальницей мы окончательно решили оставить старый план.

This is natural and neutral.

Other possible orders:

• Мы окончательно решили оставить старый план после разговора с начальницей.
• Старый план мы окончательно решили оставить после разговора с начальницей.

These still mean basically the same thing, but the emphasis changes:

• putting после разговора с начальницей first highlights the time/context
• putting старый план earlier can emphasize what was kept

Russian often prefers a noun phrase where English might use a full clause.

So instead of saying:

• После того, как мы поговорили с начальницей...
  = After we talked with the boss...

Russian can use the more compact:

• После разговора с начальницей...
  = After the conversation with the boss...

Both are possible, but the version in your sentence is concise and very natural.

Yes, Russian could omit мы here:

• После разговора с начальницей окончательно решили оставить старый план.

However, that version sounds less complete on its own unless the subject is already very clear from context.

Including мы makes the sentence explicit:

• мы решили = we decided

Russian often drops subject pronouns when they are obvious, but it also keeps them when clarity or emphasis is helpful. Here мы sounds perfectly natural.

Usually the old plan in English, because the context suggests a specific plan already known to the speakers.

Russian has no articles, so старый план can mean:

• an old plan
• the old plan

The context decides which is more natural. In this sentence, English would usually say:

• After talking with the boss, we finally decided to keep the old plan.

That sounds more idiomatic than keep old plan or even keep an old plan.