I can tell you what I would do in my attic (which is a future plan). I am in a heating dominate location with snow on the roof for a good part of the year. This plan would bring the attic inside of the insulation envelope and make it a nicer place, though likely only semi-conditioned as I wouldn't be added heating ducts. Adding more insulation on top of the current insulation will only serve to make the attic more like the exterior and less like the interior of the house.
First, I would build/install vents connecting the eve and ridge vent in every bay. This is important with snow on the roof as snow too is a good insulator. That means no matter how well you insulate the some heat will make the insulation to the snow, which means without the venting the snow insulates itself, causing some snow to melt, run down the roof, freeze into an ice damn above the eve, and likely result in water intrusion. The connected vents will carry this little bit of heat away and keep the roof deck below freezing.
Second, I would hang a second 2x4 off of the current framing using plywood supports to increase the overall thickness to allow for R-60. This also will reduce thermal bridging at the studs.
Third, I would hang rigid foam from the new 2x4 and use my insulation of choice between the roof deck and the new foam (likely cellulose here). I belieive that I would go with 2" rigid foam; this would also act as my moisture barrier (facing the warm side of the insulation).
Fourth, depending on the use of the attic, fire code, and the rating of the rigid foam a fire barrier may be needed over it. In which case I would install drywall over the rigid foam.
Fifth, I would remove all of the insulation from the bottom of the attic.
sixth, if I intend to use the attic I would insure that the framing is up to code. I already have a 2x8 framing on what would be the floor, but I believe I would need to sister them all with a second 2x8 to be up to code for a occupied area.
I'd estimate that it would cost me 6,000 to 7,000 USD in materials to properly complete this project in my house.
Note: I already have wiring in place, but if you do this you'll want to get any wiring in place once the framing is done and before the insulation starts to go in.
Also Note: This is my rough plan for a project that is probably 3-5 projects down the road.
You need to move the insulation from the floor to the rafters if you want to condition the space. Depending on what type of construction your attic is will depend on exactly how you do it. The most common household attic will have rafters going all the way to the peak of the roof with occasional ties to prevent spreading. In this situation, if you insulated the underneath side of the roof so there is no air space between the insulation and the roof, you don't need any ventilation at all. Some roofs have scissor ties which means there will be an air space between the insulation on the bottom side of the trust and the bottom of the roof and in this case you will still need ventilation. I'm sure there are plenty of Youtube videos and websites that show you how to properly insulate and ventilate (if needed) attic spaces.
It sounds like you didn't have enough airflow in the attic when you put the new insulation in and that caused the condensation on the roof and possibly mold. As long as the problem has been fixed and it sounds like that in your post, mold shouldn't be an issue anymore. If it bothers you, spray it with a bleach mixture and go about your business.
I was of the understanding the ties in the bottom third locals call them "rafter ties", which are normally the ceiling joist, prevent spreading, while the occasional ties in the upper third of the attic (seems to be every 3rd rafter here) locals seem to call them "collar ties" were to prevent lift. Have I been misinformed?