A low temp of 130F in an oil burner not rated for cold starts would ruin more than just your chimney, it would ruin the boiler. Consult the manufacturer's specs for minimum return-water temp- that would be the same low-limit you should run for the boiler in summer. (140F is usually "safe".) If you have a stainless chimney liner rather than terra-cotta or masonry liner, and cold-start boiler rated for 130F return water (unlikely) you can leave it as-is. But bumping up the min to 140F (or even a bit higher, with a terra-cotta liner) is prudent.
I have know idea what a "summer/winter" mixer is. For under $100 you can buy a fully thermostatic mixing valve and set the output to 115F or whatever it takes, and you can run anything from 115F to 215F water into the hot side, and it will mix down to 115F at the output, and if the hot side goes under 115F it'll deliver whatever temperature the hot side is, with no mixing.
For less money you can buy a tempering valve, which usually requires an input temp ~10F higher than the output temp, but works in a similar fashion.
I'm not sure what code is in PA, but ALL hot water heaters (including tankless coils in boiler) installed in my state require either a mixing valve or tempering valve at the output as scald protection. You can set the boiler/heater temp and mixing/tempering valve to whatever you like, as long as it's capable of mixing down to at 120F or lower.
At the abyssmally low (~40%) water heating efficiency in summer that you get out an oil boiler you can buy an electric HW heater, plumb it in series with the output of your tankless coil and put the mixer on the output of the electric tank, then just turn off the boiler for the summer. In summer the cost of heating with a $200 bargain-basement barely code-legal 0.90EF electric hot water heater at 15 cents/kwh (~$49/MMBTU) will be less than heating it with a 40% efficiency (water heating mode only) tankless coil in an oil boiler at $3.50/gallon ($63/MMBTU). If you're burning a couple hundred gallons or more from May-September it would pay for the electric HW heater well within the anticipated lifespan of an electric hot water heater.
For anybody heating with oil in PA it's worth considering heating at least one large zone with a ductless mini-split heat pump, since the cost will be less than half that of heating with oil. A 1.5-2 ton mini-split has enough output to heat most average-sized houses at the wintertime average temp, but not necessarily at the 99% outside design temp. But if you can heat with the mini-split at the wintertime average, it's still delivering more than half the heat (at a lower cost) even at the 99% outside design temp when using the oil-burner as the back-stop. The net result is that you end up burning only about 1/4 the amount of oil, but adding about 1/3-1/2 of the oil dollar savings to the oil bill. eg:
Say you use 800 gallons year during the heating season, and are paying $3.50/gallon for $2800 total. If you add a decent sized mini-split the oil use drops to 200 gallons/$700, but your power bill goes up by about $1000, net savings of a bit over a grand per year. A decent name-brand 1.5 ton mini-split runs about $4500, installed, a 2 ton is a few hundred more, less than $5K. If you're handy you can cut that roughly in half doing most of the installation yourself (after reading up about it), bringing in the qualified tech only when it's time to pump down the system and add the refrigerant charge.
And, you get ultra-quiet highest efficiency air conditioning out of the deal too.
In tight higher-R homes it's possible to get off oil entirely with these things, but most houses with code-min (or lower) windows & R-values will have comfort issues unless you start adding ductless heads, which can add up. But a 3-head 2.5-3-ton multi-split is still usually under $10K installed, and would be good enough for most homes, if the heads can be optimally placed.
It's a chunk of change, probably way more than what's in the coin jar on your bedside table, but so is the annual oil bill. Even with the increased US production due to fracking technology, oil still sells at the world price, and the odds of heating oil dropping back to $2.50/gallon for any period of time (where it might be competitive with the world's least efficient ductless) seem really remote to me. YMMV.