Failed Buderus LT200/2 under G215/4 — replacing with vertical indirect, sanity check on plan

Westchester County NY, 1890 house with a 2004 extension. Original hydronic install was done well; I've had trouble finding anyone competent since.

Current setup:

• Buderus G215/4 oil boiler (2004), stacked on a Logalux LT200/2 horizontal indirect (53 gal)
• Radiant: basement slab, entire first floor (aluminum plates screwed to subfloor, 3/8" PEX, ~2000 ft, oak and tile above), master bath(Heatlink manifold for 1st floor (MBR bathroom manifold and slab manifold replaced weith Upinor 6 years ago after HeatLink manifold for those loops failed.
• Baseboard: Runtal panels, 2nd floor, currently 180°F design; Wirsbo PEX supply and return from baseboard
• HeatLink 31320 injection mixing control with outdoor reset on the radiant loop; StatLink thermostats/zone modules
• Two high-temp zones: 2nd floor baseboard, DHW. Third zone roughed in for 3rd floor, not commissioned
• Three Taco 007s, all original

The failure: Tank perforated. Water along the whole jacket seam, saturated insulation, drips across the length. First symptom was a weeping drain nipple with green crusting — dezincified brass. Anode was serviced once at year 16 and came out a stub. Nobody would service it again: it's behind a 12-16 bolt gasketed flange and every contractor refused or wanted a waiver on broken bolts.

Three things I've discovered along the way:

1. No mixing valve, tank aquastat set to 150°F. Undiluted 150 to every fixture for 22 years. I assume that's what killed the brass.
2. Expansion tank teed ~15 ft (PEX, including a 7 ft vertical riser to the ceiling) from the water heater cold inlet. PRV was added ~10 years ago with no expansion tank; T&P started discharging; a tank was added at that distance and the discharging stopped — but since then all cold taps run hot briefly after a boiler cycle. Plumbers blamed hot and cold pipes touching. My read is that ~0.9 gal of expansion overshoots the tee and charges the cold distribution. Amtrol ST-12, 50 psi precharge, untested against actual static.
3. No DHW priority. The tank is just another high-temp zone.

The plan:

• Vertical stainless indirect (~52 gal) beside the boiler — leaning Lochinvar Squire SIT050, Weil McLain AquaPro (633-500) or HTP SuperStor Ultra
• Leave the dead LT200 in place as the boiler's stand, drained and capped, wet foam stripped
• Boiler does not move
• Re-tee expansion tank within a foot of the cold inlet, replace it, precharge to measured static
• Heat trap nipples; coil tied in before the mixing valve with 16" thermal drop

Possible modifications:

• ASSE 1017 mixing valve, store 140-150, deliver 120
• DHW priority via DPDT relay on the tank aquastat, NC contacts breaking the baseboard demand and the StatLink DC demand at 31320 terminals 3-4, with a timeout

Questions:

1. Anything wrong with leaving a dead LT as a pedestal long term?
2. 316L vs 444 for this — my water dezincified brass at 150°F. Preference?
3. Better way to do priority given the StatLink DC bus (or is it overkill to do this)?
4. The 31320 has a boiler-return protection setting defaulting to 135°F. The G215/4 is Thermostream with no minimum return temp. Should that be disabled?
5. Boiler is 22 years old, plate says 196 MBH input / 149 MBH net IBR, combustion tested recently (March/April). Keeping it — anything that would change your mind?

Thanks.