BURNINATE THOSE BREAKER BOXES BEFORE THEY BURNINATE YOUR HOUSE
The first order of business here is to rip every last ounce of Fire Protection Eliminated hardware out of your house and replace it with reputable panels and breakers, starting ASAP with the main panel here, as it simply cannot be relied upon to function.
I would start with a minimum of a 42-space, 200A, main breaker panel as the basis for this setup to provide plenty of space for Arc Fault breakers as well as expansion room, with a 54-space or 60-space panel preferable if space and budget allow. The feed to panel B should be rerouted to make it a proper subpanel of panel A at this point as well. Note that this work will definitely require help from the electric utility to pull the meter/shut power off to your house, and may require you to hire a professional, depending on local codes.
Once you're done with that, you can then replace the other panels at a bit more of a comfortable pace and in a more comfortable fashion as well. I would recommend 24-space if not 30-space, 125A, main lug panels as suitable replacements for panels B, C, and D, space permitting (panel B certainly can be replaced this way, while panels C and D may have to be smaller if there isn't a full-width stud bay there to fit the panel into).
Furthermore, while the breakers for major hardware seem to be correctly sized, the breakers for many of the branch circuits are improperly sized and cannot be used as guidance when selecting replacement branch breakers. This means that you will have to look at the size of the wiring on each branch circuit and use it as your guide when deciding between a 15A and a 20A branch breaker for these circuits.
Also, you may notice breaker trips after you do this due to the combination of improper breaker sizing to begin with and the noted tendency of FPE breakers to not do their one and only job. In that case, you will need to pull additional branch circuits and/or redistribute loads in order to stop the breaker tripping.
Send that tankless heater back the way it came, for it's probably not the answer to whatever your hot water woes are
This may sound strange since you already have it sitting in your house, but that tankless heater of yours is probably not the answer to whatever hot water problem prompted all this. Instead, given that your climate is reasonable for an all-electric house, you are better off using a clever device called a split-system heat pump water heater (often called an EcoCute after what they are called in Japan) that uses an indoor tank plumbed to an outdoor heat pump unit that heats the water and sends it back to the tank, where it can then be stored for use.
These are more efficient than both standard tank-type heaters and tankless heaters, and provide superior capacity for most applications (with the ability to bring water up to 180°F for storage, and then use a mixing valve to temper it down to a more reasonable temperature for use). In particular, with a tankless heater, whatever efficiency gains you get by removing storage losses in a tank from the equation are lost in the power company needing more spinning reserve running at all times in order to be able to power 112.5A of tankless heater whenever it turns on, instead of 12A of heat pump water heater or even 24A of electric tank heater. (Furthermore, modern electric resistance hot water heaters are very well insulated, with Energy Factor ratings upwards of 0.9.)
The one downside to split-system heat pump water heaters is that they require a bit more plumbing than the norm due to their split nature, but this is manageable in many installations, and is one of the keys to their efficiency -- EcoCutes do not have the "parasite load" problem that unitary heat pump water heaters do.
Other efficiency tips
If you want to get maximum performance out of what you put in for a water heating solution, there are two other things you can try. First and foremost is to insulate, or "lag", the hot water pipes in your house with at least R-4 (5/8" thick foam) or better insulation. This reduces standby losses from a well-insulated tank, and also means that the water in the pipes stays hot enough to satisfy short draws from the heater without having to wait for the heater to kick on.
The other upgrade you can make to your plumbing would be to add a drain water heat exchanger (DWHX) to your shower, dishwasher, washing machine, or other high-usage appliances. These can regain 30% of the heat lost in drain water, which is especially important if you do go tankless, to help it keep up with high-usage loads.