Is it possible to spiritbond crafting gear in combat

Nope, there is no way to end the guildleve early. The whole point of doing them is to gain you experience and gil/company seals (and later for items). That's exactly what the bonus is. For those five minutes, you should be killing as many of those creatures as you can. The experience from killing them isn't great, but what you'll get out of the bonus will make up for that handily, and is one of the more efficient methods of acquiring seals.

If you really don't want to kill more monsters, nothing says you can't just go do something else for the rest of that period. Once you complete it, you can always get teleported back to the guildleve quest giver.

There is never a time while levelling that you would "worry" about putting materia in gear. Materia is more something to edge out PvP gear or upgrade your Relic weapon to Novus level.

For PvP, there is a level 30 cap, and materia on your gear can definitely give you an edge.

For the Novus stage of the relic weapon, you can choose what stats you want on the weapon, but it requires a bunch of all tiers of materia to cap any particular stat. (Upgrading Animus to Novus)