Yes you can do it backwards but still need to achieve around 250-350 rpms for a valid compression test. Beware that some pull starter are damaged this way depending on the engine's design so you need make your not tightening the pull rope on the reverse method.
Actually compression tests on engines with an ACR is nearly useless though you can get a general idea as most will be around 60-70 psi. Though I have have seen few that would still run around 50psi most won't.
What it takes is a leak down test to determine the engine's condition via non-invasive testing. I don't even use my compression gauge on these 4 cycles anymore other than a go/no go test but I still use it a lot on the 2 cycles.
As for valve clearance you would access the valve area via breather on the side of the engine. Most require the removal of the carburetor and intake manifold to access this breather then it is a simple matter using a feeler gauge with the appropriate valve closed. Now this will not tell if the valves are sealing just the clearance. If the clearance is off then you will need pull the head to make the adjustment.
