ive done similar stuff on mine, in the process of putting my 8" f4 vixen in a hastings tube
you could also just slide the f4 primary and cell up into the f4.9 tube and test it to find where it comes to focus with an ep in it with no extension, just slide it up however many inches and use tape or any temp solution just lay it on a table sideways and eyeball the place to drill holes to mount the cell, not sure if your cell can slide in, some can, or you can modify it aor buy a used cell like an edmond or novak and they slide inside. you can later cut the tube off at the bottom if it works.
you will also have less tube flex in the focuser if the focal plane is closer to the tube...
both these newts can improve by this process, you already showed you can figure this out with your sct reducers etc, same idea here.
the hastings tubes are not much btw
also as to the mirror quality, there is some variation but mostly they are pretty decent, its not a zambutto but it is a decent mirror most likely... my orion xt12i is an awesome mirror, i have had about 8 orion dobs and all were certainly useable and decent, 2 are really good, not like my vixen but still good, and the worst of them are not bad.... based on the ones i have seen anyway.