Taking off old seam seal to replace….

Most of those seam tapes are applied with hot-melt adhesive. If yours was, tyring to get it all off is hard because you have to hit a temperature that is just enough to melt the old adhesive without damaging the coating or fabric. I would peel off all the loose tape, and then seal the opposite side of the fabric without taking all the old stuff off. Easier and safer that trying to get all the old stuff off.

I have experienced this type of failure quite a few times, but always with MSR products. Each time MSR replaced the item under warrantee. In fact, they just replaced a Hubba rainfly that was itself a replacement.

That said, assuming the gear in question is not backed by a warrantee similar to that of Cascade Designs, could you not simply pull off all the tape that was loose and use the "Shires method" of seam sealing on the outside? If any tape comes loose subsequently, you could remove with impunity knowing that the entire seam had been sealed from the other side.

I owned a TT Notch and after sealing all seams (on the outside) that thing was totally dry.

I finally removed all the old cracked seam sealer off the fly of my SD lightyear!!!!

Which is the best sealer for the job? I have some silnet, but is that not right for this material?

Given that you had tape sealing your tent , it means it has a PU coated layer on the underside so you use Seam Grip.
SilNet is for siliconised fabrics.
McNett has detailed instructions on how to prepare the tent for that and how to do it :
https://mcnett.zendesk.com/entries/21026286-How-to-Seam-Seal-A-Tent

Thanks!