That air valve is worth it's weight in gold if you're connected to one of those manic CTL Sys non-adjustable speed vacs (it stops that awful bleed-valve cycling) or some other fixed speed Shopvac type of cheapie.
As for reaffixing the abrasive to its block, some sort of contact adhesive would work well. I've used that before years ago for (non-sander) velcro repairs. A light smear on each surface, wait 20 mins until tack-free & then press together for a permanent, flexible bond. An antipodean favourite is Selley's Contact Bond, unavailable I suspect in less civilised parts or the world (joke), but I'm sure there's a local equivalent. Great for fixing loafers, brothel-creepers, sneakers & other ludicrously overpriced trainers too.
Festool's LS130 Duplex platen dimensions are indeed shared by only Festool's smallest hand block, made for them by Mirka or Indasa. It's also the same size as their Rs4, Rs400 & cordless, Mirka's smaller DEOS & OS sanders, Bosch GSS 16 & 160, at least one Rupes, an Atlas Copco/Milwaukee & probably a host of others too. I've found Mirka's DEOS @ 3mm to be the most aggressive yet smoothest handling & lightest of the bunch.
Meaning that there's a whole suite of sanding hardware & software solutions for just about every conceivable application from a range of suppliers. Mirka, Klingspor, Sia, Hermes & 3M all make well-regarded & often inexpensive velcro backed abrasives in this particular size. Mesh-type abrasives tend to be a bit more expensive, esp. the ceramic versions, but are in my experience unequalled in painting prep & intermediate (between coats) sanding. Easily torn or damaged on sharp edges, nails etc., they nevertheless tend to be longer lasting than cheaper varieties.