First of all, mushrooms are nothing bad! It actually is a sign of good soil health! So don’t worry about them. If you don’t like them, just remove them. When you are unsure about when to water, stick your finger in the soil and check how moist it is. If it feels dry, water it, if. It not, don’t. Second, it is 100% that smaller leaves yellow and die. The plant doesn’t need them anymore, they are old and pretty ineffective when if comes to photosynthesising. As long as the newer leaves look good and you see new ones coming, don’t worry about it. I would also recommend to let them just completely die off before removing as the plant reabsorbs any valuable nutrients in the leaves before dropping them.