Mainstream science does not claim that we know global sea level is rising because the Marshall Islands are being submerge by it.
The Marshall Islands are sinking because they are on the lip of the Pacific Ring of Fire, yet the Tippys blame the sinking on fictitious sea level rise. The Tippys lie.
It has been repeatedly demonstrated to you that the Marshall Islands are not on the lip of the Ring of Fire. More to the point, they are not on the edge of a subduction zone. Isostatic adjustment to sea level data due to tectonic plate movement and depression and rebound has been commonplace for over a century. Your claim on this point only indicates your unfamiliarity with the topic.
They claim that we know global sea level is rising because global measures by tide gauges and satellite altimetry show that is the case
Which is laughably obvious FUDGE given the fact that the Tippys have to LIE to "find" a "sinking island."
If you actually think that is a valid and overwhelming refutation, you're just the fool I suspected you to be.