When there's a leak between the boiler water and potable water side of a heat exchanger the flow is usually from the potable into the heating system, not conversely. Potable water systems are usually run at 30psi or higher, which is the default pressure relief setting for many/most boiler's pressure relief valves. Most heating systems are set up to run at 12-15psi.
Slow leaks over the long term would cause the PRV to be spitting water frequently, and the heating system water pressure would be running way high. If you then turned off the potable water side and opened the valves the pressure on the potable side goes to zero, and the flow would go the other way.
Sizzle is primarily hydrochloric acid, which is WAY too aggressive to use on metal plumbing on a regular basis, and is very likely the cause of your leak.
I'm surprised that the System 2000 boiler was sold without an indirect, since it's claim-to-efficiency-fame is that it purges boiler heat into the indirect at the end of every call for heat(?). Yes, an indirect (set up to use the System 2K controls for optimal efficiency) AND water softener are both in order. (Indirects can lime up in hard water situations too.)