seeding vs tabling for tempering - is the marble slab actually necessary?

Genuinely as good. Both roads end at identical tempered chocolate, and the smear test can't tell you which method made it.

Seeding: melt to 45-50°C, add finely chopped already-tempered couverture, stir until the working temperature arrives. The seed pieces donate stable crystals to the whole batch. Small space, no slab, less mess - it's the default in plenty of professional kitchens, not the compromise.

Tabling: pour two-thirds onto the marble, spread and scrape to about 26°C, return it to the warm third. Faster for big batches once you're skilled, undeniably cinematic... and completely optional.

Use what fits your kitchen. The chocolate is judged by its temper, not its choreography.

More times than you'll need - chocolate doesn't have a re-temper counter. Each cycle is just melting and re-crystallising the same cocoa butter.

What actually degrades a batch over cycles: overheating it (scorch even once and it's done), moisture sneaking in (a drop of steam thickens it permanently), and gradual thickening from picked-up crumbs and time at heat. The practical habit: add some fresh couverture when re-tempering a much-recycled batch - refreshes fluidity and gives the seed method its seeds anyway.

So re-temper freely, protect it from water and scorching, refresh with new chocolate now and then. The bin is for burnt chocolate, never merely un-tempered chocolate.