Uploading Directly to the Cloud

Yes. If Lightroom would not make a local copy, then the speed of getting the images off the memory card would be the upload speed to te cloud. And if there was a problem uploading (no internet connection or a too slow one) you would not be able to import any images at all.

I borrowed an iMac with over 1 TB of storage space and installed Lightroom Desktop. It's currently importing all of the photos (none have synced with the cloud yet). So, it appears that full size originals are imported and saved to the drive first, then synced with the cloud. Not sure yet if the originals are kept after they've been uploaded to the cloud or if they're deleted from the drive.

How many of the originals will be kept locally depends on how much of the available storage space you are allowing Lightroom to use.

Regardless of whether you just have them initially stored locally or not, keep in mind that also in Preferences>Local Storage you can change the storage location for originals to an external hard drive - it doesn't have to be the drive that the software itself is installed on. This might be a better alternative than regularly borrowing that iMac.