unlock, root, efs backup, install custom firmware from on a branded GT-I9000

Hello Folks,
the last days I have read so much, but all tutorials I read are pretty old or lead me in a dead end road.
Could somebody give me some infos what to do and how I get to a good result for my dad's Samsung GT-I9000

Unfortunately I have no windows computer here, which gives my headache with tutorials based on a windows connect. what I have:

- Mac OSX 10.10.4
- Heimdall 1.4, Kies, SmartSwitch installed
- Samsung Galaxy S i9000 (branded from 3 Austria - Hutchinson)
this phone was unlocked from a shop before, but my dad got no "unlock number". it has worked unlocked for some years. I wanted to try out if I can get into recovery by pressing "Vol.Up - ON" (no home button) and it just restarted. since then I get the popup for using "unlock PIN". even a working 3 SIM card from a friend does not work anymore. Android Eclair just shows a SIM symbol in the upper right corner with a red exclamation mark.

If possible, I want to update to a pretty new Android. I have read the post here that Coderom with Android 5.1. even runs on this model fine.
If I would like to jump from this pretty old firmware to such a new one, do I need to root, unlock it first and flash step by step newer Android version? or is it possible to flash the Coderom immediately (after root, unlock and probably a recovery installation)?

If a custom ROM is flashed, is the phone still SIM locked?

Modell GT-I9000
Firmware Version 2.1-update1
Basebandversion I9000NEJG1
Kernel Version 2.6.29 root@SE-S603 #2
Buildnumber ECLAIR.NEJG3

IMEI is shown
IMEI is shown
WLAN-MAC-Adress is shown
Bluetooth-MAC is shown

*#1234# shows:
PDA: I9000NEJG3
Phone: I9000NEJG1
CSC: I9000DREJH2

*#7465625# results in:
Netzsperre {on} /// netlock
Teilweise Netzsperre {off} ///partly netlock
SP-Sperre {off} /// SP-lock
CP-Sperre {off} /// CP-lock

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First I have tried to backup the important data from the phone (only contacts and photos).
Until now I could not backup the efs folder/partition. (probably I need root first, right?)

I could hardly connect Kies with the phone, but I have not got any data with it. And I got an initialise error if I wanted to update (so 2.2 or 2.3 was not possible). An info box showed to use Smartswitch.
So I have used Smartswitch on the Mac and could connect via USB to the I9000. I have seen public folders and made a backup of the contacts and pictures. An upgrade was not possible too. Thus, the 3 branded Android Eclair is still on the phone.

With "Vol.Up - ON - Menu" boots the recovery.
With "Vol.down - ON - Menu" boots the yellow construction figure state for Odin. (download)
No play-store on the phone. Only google market (where I have not found anything useful). I have searched on apkmirror and could not really find apps for Android 2.1.

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what would you suggest to do and in which sequence? (without windows)
- root
- unlock
- backup efs folder
- install clockwork mod
- format somehow f2fs ?
- flash coderom

thanks for any help!
Michael