I look for printer/ink/media combinations that meet my personal requirements for lightfastness without needing any additional top coat. Once I have identified an ink and media combination that achieves that objective (typically passing Aardenburg Conservation display rating limits at greater than 100 Mlux hrs), I believe the weak link for image permanence then moves to other factors such as humidity resistance, thermal stability, and resistance to cracking and/or delamination over time. Any additional light fade protection achieved by further treatments is welcome but not necessary in that situation unless it can be shown to beneficially influence the other properties as well.
Polymer sprays (e.g. Hahnemuhle Protective Spray) certainly increase the resistance of prints to scuffing and physical abrasion, as well as improving lightfastness.
With traditional paintings, varnish coatings are both decorative and protective. When they finally discolor of fail in any way, they can usually be successfully removed by a paintings conservator and replaced. Inkjet prints have far too delicate an image receptor layer to stand up well to traditional conservation treatments, IMHO, so it's a one shot deal when you apply any top coat to an inkjet print.
Physical durability is one of the things that concerns me most when it comes to 'archivability' of photos, which is why I think mounting to Dibond is generally a good thing, despite its irreversability, and have become more and more interested in the potential of direct-to-substrate UV printing (which is both extremely lightfast and physically durable, when a UV-curable topcoat is added). After all, there are countless examples of images (photos, paintings, etc.) where the image itself is intact, but the substrate is falling apart or otherwise extremely fragile.
Following on an idea from here a few years ago, I've had a lot of success spraying Breathing Color Pura Velvet and Pura Smooth, as well as Moab Entrada Natural, with Timeless - diluted 5 parts Tiimeless with 1 part distilled water, and with a drop of Photo-Flo (optional), it soaks through the receptive layer and deep into the paper base. When it hardens, the image, receptive layer and a good portion of the paper is all bound up into a single, thick layer of Timeless - it doesn't tear, and, even if you try to delaminate it by applying duct tape and ripping it off, the delamination occurs deep in the paper base (i.e. where the layer of Timeless/image/paper fibres ends) and not in the receptive layer itself. 1 coat for an almost-matte look, 2 coats for a satin finish, 3-5 coats for high gloss (5 coats is almost mirror-like). Doesn't seem to work as well with Canson papers. Fortunately, the Breathing Color papers - at least, the Optica One and Elegance papers you've tested (which are basically Pura Smooth and Pura Velvet with OBAs) and the Entrada Natural seem to have among the best longevity of any paper with any given inkset (just comparing the test results with Ultrachrome K3). Haven't tried it with gloss papers - maybe a 1:1 or 2:1 dilution with a drop of surfactant would allow it to penetrate even glossy fibre-based and baryta papers (there's no penetrating RC papers, though). In any case, you can get any level of gloss you like with the Timeless on matte paper.
Have you done much with the Kernewek/Arista Americana fabrics? I've had the chance to see a few prints made on them, but haven't been able to print on them myself. They're not like a normal canvas - up close, you see an even mesh of fibres, not a coating of gunk that can crack and peel off. Instead, the receptive layer is soaked directly into the canvas fibres and can't delaminate, and your tests seem to indicate they have excellent lightfastness - even better than Lyve. Would like to use it, but, at the moment, I don't have a functioning printer...
