1. Use the Grand Celestial (GC). It is significantly better than the others (Attack Power is usually a good indicator of which weapon is best). Ascendant is an old, baby pvp weapon, and Rift would need a lot of upgrading to catch up to your GC. So scrap them and keep upgrading the GC. With it, you should be able to do your basic dailies; just be prepared to be stuck at stage 9 for a long time (due to that breakthrough item).
2. The first 3 red quest lines (Raven, Hive Queen, Emperor) have solo versions of the boss fight. After the NPC ports you in, the boss should be fairly easy to solo, so if you're dying you might be using really outdated gear (like the Ascendant weapon) or need some practice with your class. Fortunately, these quests only give you achievements and a few items, so you can always come back and do them later if you aren't able to now.
3. Unfortunately this game requires a lot of different items for upgrading and it's hard to know what all you need now that the equipment progression window is currently unavailable. Any kind of Soulstone, Moonstone, Elysian Orb, Sacred Orb (or their crystal versions) should be kept. Scales and Elements are also needed. Otherwise, without knowing what items specifically, I can't say. But the cost of expanding your inventory is quite cheap now, so I recommend doing that if you haven't.
4. As mentioned, the upgrade tree was disabled with the UE4 patch. Will it return? I don't know, but I hope so.
5. Sadly if you have an older computer or slow internet, this game can be pretty unforgiving. You could try warning your team mates in advance that you're a slow load, but many will probably not want to wait. Koldrak's is an easy daily, but you may have freezing lag there. MSP is also an option. You'll need to find an alliance, but only one load screen to go through then you stay inside. Otherwise, they have taken away most of the content solo-able by low levels.