No, a magazine for a Jimenez J22 will not fit a Jimenez J25 pistol. While both pistols are manufactured by the same company, they are designed for different calibers and have distinct magazine dimensions. Therefore, using a J22 magazine in a J25 could lead to feeding issues and is not advisable. Always use the correct magazine specifically designed for your firearm model.
Yes, a Jennings J22 will fire without a magazine inserted.
These retail for about $85 new.
That's a task best left to a gunsmith.
frame
Best thing is to have a gunsmith show you.
Recommend you ask a gunsmith for help
The thing that keeps the slide on a j22 frame that won't stay on is the slide catch. It is designed so that the slide mechanism can be locked in place without risk of it moving.
Best to ask a gunsmith for help if you don't have a manual
.22 LR rim fire
They were about $60 new, and now about $100 in good condition at a gun show.
$50 to $75. These were very crude, low cost firearms of no particularly high quality.
You will have to experiment with as many different brands as you can afford to find out which it likes.