The choice between ext4 and btrfs is available in Calamares (the Manjaro installer) – the default is btrfs – I can only presume you missed it.
No, though some prefer to wipe their disks in preparation, but it’s generally unnecessary as Calamares will apply the file system of your choice when the “Erase Disk” partitioning method is used.
This might be highly impractical even though possible – and you still need to download the updated ISO to create a new Installer USB (or burn it to DVD).
If you have access to another computer – a family member, a friend or co-worker – then you can produce a Manjaro installer on that computer (even if it’s Windows). I suggest creating a Ventoy USB.