The converter supports both, choosing only ARMv7 doesn't make any difference but only not showing "Conversion Failed" due to ARMv7 syntax is different.
Similarly, if you put a ARMv8 instruction there you will get an error and first 2 row will be empty.
Should not be a problem, the developer put an option there for convenience
It SHOULD fail. The ADDS syntax given is only limited to ARMv7 (32bits) not v8 (64bits). The converter is not able to convert a 32bits into 64 since the 2 are fundamentally different. If you are just modding armebiv7, all you need is ARM HEX value, not ARM64 HEX value