If you're unable to find the royal crossbow, think about OSRS gold a think about getting one like the Armadyl. Always utilize 2H or dual weaponry for slayer tasks. Go with the highest tier weapon that you have. If you got a lv 80 ranged wep and an 80 melee weapon as the top weapons, use the weapon which your monster is the weakest (according to combat triangle).
You'll never need more than bandos/ barrows for slaying tasks. If you're in need of lvl 80 weapons I would suggest to go for chaotic weapons, and save your cash on other items. I would recommend that people not buy things at level 90 if they've got money to burn. That's entirely up to you.
For bolts, you may use any bolts you want on any bow. The compatibility thing was removed. If you use level 80 royal bolts along with lv 70 crossbow, the bolts will have damaged caps just like the bolts were at level 70. If you own a level 60offhand bow the mainhand bow will be capped at lv 70 bolt damage, while the offhand bow will cap at the level 60 damage. However, I do purchase Royals as they're cheap and it's not really a big deal either way.
Ganodermics do pretty well with the majority of things that are magical. In my beta test the game, and when you die they won't really lose charge this is a bonus. Beware of weapons that were previously equipped with special attacks since those attacks are back - check the Combat Beta for more information.
The products that have added critical rates are pretty useless IMO. It's more about precision and buy runescape 3 gold base damage than critical numbers. As an example, a fire cape isn't like the skillcape IMO.