Bu arada, yaya geçidinde bekliyorum; toplantı kaydını telefondan ben başlatıyorum.

Both are possible.

• As “by the way,” Bu arada introduces a side remark or a new, loosely related topic.
• As “in the meantime/meanwhile,” it situates the action in a time window overlapping with something else.

Here, given the ongoing actions, either nuance can work. If you want strictly “during this time,” you can also use Bu sırada or Bu esnada, which are more purely temporal and less “topic-shifting” than Bu arada.

A semicolon in Turkish works much like in English: it links two closely related independent clauses. You could instead use:

• A period: Bu arada, yaya geçidinde bekliyorum. Toplantı kaydını telefondan ben başlatıyorum.
• The conjunction ve (and): Bu arada, yaya geçidinde bekliyorum ve toplantı kaydını telefondan ben başlatıyorum. All are acceptable; the semicolon just keeps the tight connection without using a conjunction.

Because yaya geçidi is an indefinite noun compound (“pedestrian crossing” as a fixed unit). In such compounds, the second noun takes a 3rd‑person possessive suffix. Then you add the locative case to that form:

• yaya geçit
  • 3rd‑person possessive -i → yaya geçid-i
• plus locative -nde → yaya geçid-inde

So it’s not the bare noun geçit with the locative; it’s the compound yaya geçidi with the locative on top.

The locative is indeed -de/-da (or -te/-ta after voiceless consonants). However, when a noun already has a 3rd‑person possessive ending (-ı/-i/-u/-ü), Turkish inserts a buffer -n- before the case ending. So:

• geçid-i (3rd‑person possessive) + -de (locative) → geçid-in-de = geçidinde.

Final consonant voicing: when a suffix beginning with a vowel is added, final t in many words becomes d.

• geçit → geçidi (because of the vowel-initial possessive -i)
• Similarly: kitap → kitabı, kanat → kanadı

It’s the present continuous: verb stem + -iyor + personal ending.

• bekle- (to wait) + -iyor
  • -um → bekliyorum = “I’m waiting (right now).” For habitual statements, you’d use the aorist: beklerim (“I wait/I usually wait”).

Two things are stacked: 1) toplantı kaydı is an indefinite noun compound (“meeting recording”), where the second noun has a 3rd‑person possessive: kayıt → kayd-ı. 2) Because it’s a definite direct object, you add the accusative -(y)ı/-(y)i/-(y)u/-(y)ü. After a 3rd‑person possessive, you insert buffer -n-: kayd-ı-nı.

So toplantı kaydını = “the meeting recording” (as a specific object) with accusative.

• telefondan (ablative “from/through the phone”): you’re initiating it via the phone, from the phone as the source/device.
• telefonda (locative “on the phone”): usually “on a phone call” or “on/at the phone” (location).
• telefonla (instrumental “with/by phone”): “by phone” (e.g., contacting someone by phone).

Here, telefondan fits the idea “from the phone (app/device).”

Yes, it’s natural. In Turkish, focused/emphasized elements tend to appear right before the verb. Some options:

• Ben toplantı kaydını telefondan başlatıyorum. (emphasis on “I”)
• Toplantı kaydını ben telefondan başlatıyorum. (stronger emphasis on “I” doing that specific thing)
• Toplantı kaydını telefondan ben başlatıyorum. (also emphasizes “I,” after another adverbial)
• Toplantı kaydını telefondan başlatıyorum. (emphasis on “from the phone”)

All are grammatical; the nuance depends on what you want to highlight.

Yes:

• Immediate/ongoing: başlatıyorum (“I’m starting it [now]”)
• Planned future: başlatacağım (“I will start it”)
• Habitual/neutral statement: başlatırım
• Suggestion/voluntative: başlatayım (“Let me start it”)