Dry hopping in primary is totally fine, I do it all the time. It does the exact same thing. The main reason for secondary is getting clearer beer, but I never do secondary as it increases the chance of oxidization and infection. I get very clear beer without secondary just by cold crashing and letting it sit for a while. After kegging/bottling, just let it sit for a while. Most of the remaining trub/hops/yeast/etc will settle to the bottom.
1) 6 weeks in primary is very safe. When you say will the yeast start to die, are you worrying about autolysis? 6 weeks is not very long, I've never heard of anyone having that problem for any length of time. I think it's something homebrewers used to worry about a lot, but it's not really a common problem. Lots of people leave beer in primary for months at a time, yeast and all. In How to Brew, John Palmer says:
As a final note on this subject, I should mention that by brewing with
healthy yeast in a well-prepared wort, many experienced brewers,
myself included, have been able to leave a beer in the primary
fermenter for several months without any evidence of autolysis.
But you do your dry hop at the end, just before bottling/kegging, for however long you're doing it. I would say anywhere from 24 hrs to 7 days is good. Some people go longer, but you're going for freshness here and too long can also impart grassy or vegetal flavors.
2) You generally don't want to dry hop while there's any krausen. Krausen is a sign lots of CO2 being produced and bubbling out of the beer. If CO2 is leaving the beer, so is your hop aroma. The point of dry hopping is to increase your hop aroma, so you don't want it bubbling at all anymore by the time you add your dry hop addition, because this will decrease the effectiveness of the dry hopping as the CO2 will carry away the aroma molecules. After about a week in primary for most ales, the foam should settle and dissipate.
You can make an excellent, hoppy IPA and Pale Ales by simply fermenting for 7-10 days, then dumping in the hops and leaving for another 2-7 days (or less), giving you a full 2 weeks in primary/conditioning. Then bottle or keg and condition for a week or two. You can pour the hops right into the fermenter but you want to be careful when transferring to keg or bottling bucket that you avoid transferring them. You can do this by leaving the siphon/racking cane above the trub line, or putting a filter on the siphon (e.g. a strainer bag). Alternatively you can dry hop in a mesh bag or other straining device. Personally I don't do this, as more contact with the liquid increases hop utilization, but lots of homebrewers use mesh bags or baskets or tea balls to make straining them out easier.
Also I'm not sure why you would worry about "holes in your krausen". Dropping some hops on the foam would not rupture the yeast cells floating around in there.
How long can homebrew stay in the primary fermenter?
Too long in Primary Fermentation?