The sealant is anerobic in nature which means it only hardens in the absence of O2. However, leaving it hanging open like that is a bad idea. Tighten the thing down and be done with it so it is sealed up and nothing will interfere with the process. We could theorize for days about what could happen, spark plug O ring could roll over, a little oil could drip from God only knows where onto the gasket area and cause it not to seal, a bug could crawl across it and disturb it, wind blown b.s. in there. The bottom line is it's not meant to be left hanging half way done, bolt it down tight and be done with it. As far as the cam timing goes it is obviously best if you do it the correct way and use the Volvo tools but if you do not have them then I guess you have to wing it, good luck.

Mark