Design Review - heat pump water heater with boiler coil redux

Hello All and thanks in advance for any thoughts you might have on the following approach to installing a hybrid heat pump water heater (HPWH/HP) that will hopefully become the main source of domestic hot water for a system that currently relies on an oil-fired boiler coil. I've followed many of the discussions on this general topic here on Heating Help and have hopefully used what I have learned to design a "mongrel" install that addresses the concerns raised when bypassing (but not removing) the coil. First some context: I live in southern Maine, so generally considered a cold climate though things are changing, summer is longer and hotter, winters shorter and cold but less predictably so. Current heating system is a 90's vintage Utica boiler with a Beckett burner and forced hot water baseboard distribution system. It's had fairly light use, runs reliably and still holds consistent temp and pressure, but obviously it doesn't owe me anything and I'm gradually preparing to replace it with an air to water heat pump system when she finally gives up the ghost. I run it at ~180 F on the high end, which still gives the two of us plenty of heat and hot water. We are conservative with resource use and have a wood stove to take the edge off in winter.

Here's the issue: I purchased a 50 gal. State HPWH, with a generous point of purchase rebate from Efficiency Maine and intended to use that exclusively for DHW. Low and behold, when reviewing the install instructions I discovered that the heat pump mode will not operate if water temperature in the tank is below 59 F. I knew about the HP cut out if the ambient air temperature is below 45 F (not a problem, it's in the basement next to the boiler) but I was unaware of the 59 F cut out for the tank water! In either case, if the temp is lower than the low default, the unit switches to the electric heating mode, which is not a step up in efficiency. We have a drilled well and the water temperature fluctuates from a high of ~52 in summer to a low around 45 F in winter, 7 - 14 degrees below the cut out year round. This is a problem since I purchase the HP to reduce our oil consumption (particular in the summer) and as part of an effort to improve energy efficiency. Though I am uncertain of how long the unit will have to operate only using the electric elements to raise the temp enough for the HP to kick in, time and experience will tell, I've decided to attempt an install that will allow use of either the boiler coil or the HP. The design I am proposing will allow me to isolate, drain and vent the boiler coil when using the HP and vise versa, if it turns out to be necessary to continue using the coil during the coldest months. I would appreciate any comments you might have on the functionality and safety of the proposed design and I will do my best to answer any questions.

I have attached three photos that hopefully describe the design in sufficient enough detail for reviewers to understand how the installation will work. I'll let them do the rest of the talking.

Thanks for any help,

Rusty