Heaters have a maximum flow rate, if you exceed that it is possible to damage the heater. Normally the max flow rate on the heater is higher than your pump can produce, so there is no issue. But in rare cases a very large pump could exceed the max flow rate on the heater and to avoid damage you would need to use a bypass valve. The only other longevity related issue would be bypassing the heater when your chemistry is way way out of balance, which will hopefully never come up, but can be handy if you actually need it.
Heater corrosion is almost entirely a function of water chemistry getting way way out of balance. A SWG does nothing to promote corrosion of heaters in and of it's self. Copper heat exchangers can be damaged by low PH, which isn't something a SWG is going to cause but does tend to come up with trichlor tablets. All heaters can also sometimes be damaged by calcium scaling, which is really a chemistry problem though a SWG might contribute to scaling conditions to some extent.
Most heaters have internal bypass setups which operate automatically. External bypass setups are getting a little more common than they used to be, but are still fairly rare. An external bypass can be automatic or manual. Most of the ones I see are manual. Manually controlled external bypass setups are getting more common because they can improve the overall pool system efficiency (i.e. save electricity) when the heater is turned off and bypassed.
