The PB Blaster mentioned earlier has worked well for me. Any good penetrating oil should do the trick but PB Blaster is high on my list.
The fasteners on the tips may be there forever (unless you break them off, ofc). They are exposed to hot, corrosive gases mixed with water and tend to become VERY corroded. Use some anti-sieze when you replace them (only takes a tiny bit) and they should be easier to remove next time.
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