Android - How to resume app restore after stopping it on first opening Google Play?

Use bmgr from adb shell. See this answer for a complete explanation of backup sets.

Root access is not required.

shell@maguro:/ $ bmgr list sets
  1234567890abcdef : droid48
  34567890abcdef12 : galaxy49
  ...

shell@maguro:/ $ bmgr restore 1234567890abcdef
Scheduling restore: droid48
done

This triggered a full restore of all the previously installed apps on my device instantly.

There's no option to resume. If you stop you can only manual download later.

If you didn't pressed stop, you could just disable data transfer and, when connected again, download will continue.

If you stop, the current sync state will be sent to the server and it will be (without the apps that weren't installed) stored as the device final state after restore.

And by the way, clear market data will not help (Tested >5 times on reflash my Desire)

If you had turned off 3G, or even the device altogether, the downloads would have 'paused' so to say, and would have been resumed as soon as a data connection became available.

Apart from that, there is no way to pause an app download from Google Play, to resume later.