Help- Remington model 572

A friend brought me a basket case 572 which was his grampa's and he wanted me to put it together and replace the missing parts, which I did and it functioned perfectly. I replaced the forend along with the escutchen, nut and screw, the mag tube screw. Not much. I didn't want him to shoot it before I checked it out to be sure it was safe to fire. I took it to the range yesterday and shot it, I had about two dozen light strikes but not jams, sights were off alittle but I cut paper. I got home and cleaned it and noticed that the when assembled and with the barrel pointed up, the forend would fall and the breach would open. The action bar threads were stripped which stripped the new forend screw. I tapped the action bar to 10-24 and put a new screw in the forend. I am thinking that the action bar is worn out, could this be the problem with the breach not locking shut?
As for the light strikes, I am thinking that the hammer spring is too weak, am I right? The hammer looks a little mushroomed also so would it be a good idea to change the hammer? Even though he said that he probably would not shoot it because it was his grampa's, I do want it to be safe and function like it should. Can anyone help me figure out why the forend falls open when pointed upward and why it is suffering from light strikes. I used three different kinds of .22 ammo and had light strikes with all. Pleas help,and thank you in advance for any and all information on these problems.

PS. If these are the fixes for the problems I can order the parts and replace them but if not then he will have to take it to a gunsmith, which I don't claim to be.