I successfully applied from South Korea under the category “workcation” and I was not asked for a lease agreement or invitation letter - what are you applying under? Workcation or soft power?
Re: certificate of employment, is it a company you work for, not one you own? Reason I ask is I’m assuming that the expectations will be different in each case.
You enter as many times as you like - and each time you enter, you get stamped in for 180 days, which you’re allowed to extend a further 180 days at immigration. “Once” means you can’t keep extending at immigration, you can only do it once, then leave and come back.