Regarding the first question, if you adjust your estimated tax payments as you describe, to avoid an underpayment penalty for the earlier quarters you'll need to annualize income on Schedule AI of Form 2210, an onerous task. Unless interest rates rise substantially, it's generally not worth the effort to do this instead of just paying 25% each quarter. However, if you track your income throughout the year, in anticipation of preparing Schedule AI, it might not be so bad. Still, the amount of time you spend on that could easily result in a doubling of the effort needed to prepare your tax return.
The tax system is pay as you go, so underpayment is determined quarterly. The tax code treats your income as received uniformly throughout the year unless you show otherwise on Schedule AI. I imagine that California does similarly.
Regarding the second question, if you pay part of your required (based on the particular method of calculation that applies) Q2 estimated taxes in July instead of by the June 15 deadline, they will be late (absent any change in the deadline as we have seen in recent years) and the penalty will be determined by the number of days that the payment is late. The calculations are done quarterly, not monthly. See Form 2210 and its Schedule AI for details.
