recovering data from an imac 2017 with mojave that won't boot

Your ability to boot to Recovery mode suggests that Target Disk Mode may work. You did not indicate whether you tried to access your data volume via Terminal in Recovery, nor whether you tried to get to Single User Mode (CommandS during boot) so we don't yet know how intact the filesystem is.

Given that the root cause of your issue is unknown, we should be conservative and presume that your data is in imminent danger of permanent destruction. This means that the top priority is copying as much of it off the drive as quickly as possible. This seems to be in alignment with your goals.

If there is no or minimal filesystem corruption, then Target Disk Mode will generally be successful in allowing you to siphon your data. If corruption is significant, you may need to run Disk First Aid in Recovery first and hope that it can repair the damage. If it cannot, then you will need to use a service like DriverSavers, which to my knowledge is the only shop that's capable of recovering APFS volumes. (I am presuming that your data volume is in APFS format, which is the default for Mojave. If it is instead HFS+, then you can often use DiskWarrior to repair it.)

So the answer to both Q1 and Q2 is: It really depends on if & how much your drive is corrupted.

Important Note: The more time that the computer is powered on, the greater the risk of further data loss. Leave the system off until you're ready to try.