How hard shouldn it be for the lifters to collapse. I have crane hydraulic lifters and when I am adjusting my valves it is almost like mechanical lifters, they are very solid. If I tighten the nut it will push down on the valve. The best i can do is zero lash.
If you have hydraulics, that's where you want em-- zero lash. While tightening them, spin the pushrod with your fingers, when the pushrod gets tight, tighten the nut 1/4 to 1/2 turn more, then you're done.
They only need .010. You just need to get the plunger off the clip in the lifter. D.Hearne way works. I like to push on the back of the rocker to 0 lash and hand tighten the nut down. Then give it the 1/4-1/2 turn. The last 1/4-1/2 turn is where you get your preload. They are very hard to push down when they are pumped up. Don't worry about it.
Have you already tightened them too much? I have always been able to push the lifters down by hand when properly adjusted. If they were tightened down beyond proper adjustment to bottom out the lifters, then next thing to open would be the valve--which is what you said happens.
I would back all of the rockers completely off and start over with the tightening process by following the spinning pushrod method previously described to set the preload. Better yet, get a dial indicator and set the preload with precision.