1. How about ledger nano x which support bluetooth, can it be affected by malware through bluetooth while performing online transaction?
You compare it to any others hardware wallets way to move unsigned transactions onto the hardware wallet and the signed one to the broadcasting device.
The one big difference is that it is a wireless technology.
So generally speaking, the risk is larger since an adversary doesn't have to be right in between your hardware wallet and your broadcasting device (e.g. with a manipulated usb cable or by compromising your device), but within a few meters.
This only really plays a role when there is a vulnerability in how the hardware wallet handles (weird) requests etc.
So theoretically, yes. The attack surface and therefore the risk is larger.
But practically this is / should be negligible.
2. Is it possible for hardware wallet to remain totally offline, while using another wallet as watch-only wallet in a way the hardware wallet will be used offline only for signing through the means of QR code?
It depends on how you move the unsigned/signed transactions between the HW wallet and the broadcasting device.
If it has an inbuilt camera, yes.
If via USB from an offline device, yes (but not using QR's).
If via USB from an online device, not so. Remember that the HW wallet never really is online, but also isn't completely offline when plugged into an online device.