Problem could be flapper valve with seat. Turn off water. If water continues to drop a couple inches with time, I suspect flapper or seat. Feel seat. Replace flapper.
Hard to tell. Is the water level all of the way up to the top of the overflow tube? It should typically be a half inch lower, but definitely not to the top.
Your refill tube looks like it is held above the overflow and does not dangle into the overflow. If that is the way it is, that is good. A dangling refill tube could also show a problem.
If you can tighten the tank bolts by hand or gently with a wrench, that is OK. However that is unlikely to be the cause of your symptom. Loose bolts cause tank wobble and leaks outside the toilet usually.
These things will not fix a weak flush. See if holding down the handle during the flush helps give a stronger flush.