Could <PERSON> have safely used the Ancient's Repository of Knowledge?
Well, it depends. When are you talking about? If you're specifically thinking of after his "descent" back into the mortal realm, probably not, although there is no reference in the series or movies that provides a good reason beyond what was already established for <PERSON>. Others have mentioned that <PERSON> probably didn't have the ATA gene, but I counter that with the fact that the 1st Repository reacted to Teal'c who most definitely didn't have the ATA gene, either.
But...to take them in order: The first time (when <PERSON> used the knowledge to dial up Asgard): probably could have used it, just not safely. Probably would have had the same outcome as it did for <PERSON>. If you notice, the "translation" services provided by <PERSON> don't really change <PERSON>'s behavior in any meaningful way, except to allow the team to come with him. <PERSON> could have techno-babbled a translation matrix to accomplish the same thing while <PERSON> was still coherent.
The second time: same thing, again, as <PERSON> still hadn't ascended.
The third time: this was <PERSON>'s consciousness, not the full repository of Ancient Knowledge. The episode clearly indicates that it is a subset of the Full Repository and therefore should take longer to completely over-right an individual's consciousness. 663
<PERSON> clearly had no issues with it, as he was an Ancient, and it was his repository. When <PERSON>'s mind merged with <PERSON>'s, it still had the power to control the amount of information being "unspooled", but even that had it's limits and essentially provided a "count-down" timer to when <PERSON> would have to leave <PERSON> and take the knowledge with him, thus effectuating the same kind of rescue that the Asgard provided <PERSON> with the 1st Repository. Not exactly safe, but not highly dangerous, either.
As for why he didn't try: because sometimes knowing that you don't know if all that you need to know. Pre-Ascension <PERSON> saw the damage it did to <PERSON> and the reasoning of being a translator for him was good enough. Post-Ascension <PERSON>, well...it could be any number of reasons, from fearing a reprisal by the Ascended to fearing remembering what he knew when he was Ascended only to lose it all again, feeling safe in knowing that he once knew and would probably know again, feeling safe in the idea that the knowledge was too much for humanity to have all at once (see also the <PERSON> [sp?] episode), being accepting of the notion that sacrificing oneself (or appearing to) was something that meant more to <PERSON> than it did to him. There is no prime canon explanation. Beta canon may have something, but I avoid beta canon like the plague, so...there ya go 865