كل من الجلسة الافتتاحية وحلقة النقاش في قاعة المحاضرات.

It is a very common pattern meaning both X and Y or each of X and Y.

So:

• كل من الجلسة الافتتاحية وحلقة النقاش = both the opening session and the panel discussion

Here, من is part of the pattern. It is not functioning with its usual everyday meaning from.

Because in Modern Standard Arabic, the present-tense is/are is usually not stated in sentences like this.

So Arabic can say:

• كل من الجلسة الافتتاحية وحلقة النقاش في قاعة المحاضرات

and the meaning is understood as:

• Both the opening session and the panel discussion are in the lecture hall

The phrase في قاعة المحاضرات acts as the predicate, so the sentence is complete without an explicit verb.

If you changed the tense, you would normally use a verb, for example in the past:

• كانتا في قاعة المحاضرات = they were in the lecture hall

Because Arabic adjectives normally come after the noun they describe.

So:

• الجلسة الافتتاحية
• literally: the session opening
• natural English: the opening session

This is the normal Arabic order:

• noun + adjective

Because Arabic adjectives agree with the noun they describe in definiteness as well as gender, number, and case.

Since الجلسة is definite, the adjective must also be definite:

• الجلسة الافتتاحية = the opening session

Compare:

• جلسة افتتاحية = an opening session
• الجلسة الافتتاحية = the opening session

Because حلقة النقاش is an iḍāfa construction, often called a construct phrase.

In an iḍāfa:

• the first noun usually does not take ال
• the second noun determines whether the whole phrase is definite

So:

• حلقة النقاش
• literally: session/panel of the discussion
• natural English: the panel discussion or the discussion panel

Since النقاش is definite, the whole phrase is definite.

Also, حلقة literally can mean circle/ring, but in contexts like this it often means a session, panel, or discussion group.

Yes. قاعة المحاضرات is also an iḍāfa.

• قاعة = hall
• المحاضرات = the lectures

Together:

• قاعة المحاضرات
• literally: hall of the lectures
• natural English: the lecture hall

Again, the first noun قاعة does not take ال, but the whole phrase is definite because the second noun is definite.

It applies to both items.

The structure is:

• كل من X وY = both X and Y
• في قاعة المحاضرات = in the lecture hall

So the meaning is:

• both the opening session and the panel discussion are in the lecture hall

Arabic does not need to repeat the location after each item.

The important endings would be:

• كُلٌّ
• الجلسةِ الافتتاحيةِ
• حلقةِ النقاشِ
• قاعةِ المحاضراتِ

Why:

• كُلٌّ is nominative here as the start of the subject phrase
• after من, الجلسةِ is genitive
• الافتتاحيةِ matches الجلسةِ
• حلقةِ is also genitive because it is coordinated with الجلسةِ
• النقاشِ is genitive as the second noun of an iḍāfa
• after في, قاعةِ is genitive
• المحاضراتِ is genitive as the second noun of another iḍāfa

In normal unvowelled Arabic writing, most of these case endings are not shown.

Because كل من ... و... is a very common and straightforward way to say both X and Y when you explicitly name two separate things.

كلا and كلتا can also mean both, but they are used in different structures, often with a dual noun phrase, for example:

• كلتا الجلستين = both sessions

When you are listing two different coordinated items by name, كل من ... و... is usually the easier pattern to use.

Yes. Arabic allows some flexibility in word order.

The given sentence has a neutral order:

• subject first
• then the location

You could also front the location for emphasis:

• في قاعة المحاضرات كل من الجلسة الافتتاحية وحلقة النقاش

That would put extra focus on the lecture hall. But the original order is the most straightforward one for learners.