Is it OK to reduce the charging current for a Li-ion 18650 battery?
Yes it is OK to reduce the charging current and that is what the TP4056's charging current setting resistor is for. If you halve the charging current, charging will of course take longer. Charging slower (but not too slow) should actually increase battery lifetime. But for an 18650 cell, 1 A charging current is reasonably normal.

2.8 V is a very low voltage for a Li-Ion cell, if you discharge often to such a low voltage the battery's lifetime might be shorter. I would prefer not to discharge below 3.4 V.