1. If it looks like epoxy
cast 'bushing' then it may be one. Chip it out with a tool. Do NOT bang the plates from below trying to push them up - you may lift
2. Non-technically-wise, are you making a DECISION? Good. There is only 'either do or do not'. My young padawan. Your focus defines your reality.
3. Attached: image of the chainplates we replaced this summer. Age: 10 years. Material: SS. Miles: 20k. Boat
sailed intensively and kept outside in a tropical climate.
PS If you replace your chainplates, you know
you have 100% good stuff. If you do not, you may believe
you have 100% good stuff.