Buffer tank control strategy

Meeting this afternoon with sister's GC and HVAC guy to discuss her Burnham 206 gas boiler running a mix of gravity conversion with original large pipe and cast iron rads plus newer baseboard piped in 3/4" copper.

House is 3300 sq ft with 110,000 BTU/hr design heat loss. I did the manual heat loss, informed by actual gas consumption last season. Uninsulated concrete basement is an envelope problem that would benefit from rigid foam board, and I will mention this as an option for improvement.

There are also a number of system improvements that could be made for better efficiency and control. Auto vent damper is one. Buffer tank is another, as boiler is probably short-cycling due to over-zoning (6 total zones, 4 of which on one floor!) And a buffer tank would help solve other problems, such as a cast iron radiator in a distant upstairs bedroom repiped with 3/4" copper, at the end of a gravity conversion loop, so it gets no flow. Only solution I can see is to add a circ to that one 3/4" copper loop, making yet another zone which is worse for efficiency unless we add a buffer tank.

So I'm trying to understand optimum control strategy for a buffer tank, in case that turns out to be an option we decide to pursue. @hot_rod helped with initial tank sizing. 80 gallons looked OK but I decided to do the math for 120 gallons because a larger tank isn't much more expensive and it helps increase run time. So I drew up two different scenarios based on (1) full mixing within a 120 gal buffer tank so that the water inside is always at or close to average temp, and (2) stratified so that hottest water stays on top and coldest on bottom. I understand that stratified is best for radiators, because they will get the hottest possible water from the top of the tank. However, isn't stratified also worse for boiler efficiency, because now the boiler supply temp needs to be higher in order to reach the higher temp at the top of the stratified tank? Note that in the stratified example, boiler SWT has to end at 170 instead of 160 in the fully mixed example.

These two pics illustrate the two different cases, fully mixed tank vs. stratified tank. Both case represent the minimum run time case starting with 120 F avg water temp and ending with 140 F avg water temp, for a total input of 20,000 BTU into the tank from a 9-minute boiler run. So 9 minutes would be the minimum boiler run time. ODR would raise the target average water temp higher than 140 in colder weather, but 140 would be the minimum target temperature in warmer weather. And the boiler would have a thermostatic bypass set to 130 minimum.

So here's my understanding of how that would work.

So my questions are:

1. Are these schematics more or less correct?
2. Which scheme is better in this situation?
3. Is the variable speed circ in the second schematic the right way to get higher water temps needed for a stratified tank?
4. Is the tank middle the best place for a temp sensor, since it will give a tank average temp regardless of stratification or full mixing?

There is also the option of full replacement with a mod-con which would solve all these problems, but she doesn't know how long she'll stay in the house, so up front cost is a factor. She could save 30% by spending $20k on a new mod-con, or maybe 15% by adding a vent damper, increasing Tstat swing and adding a buffer tank for maybe $10k. She'll have to decide which way to go.