Expeditions: what is this, and can I play it on 1.2.8?
Expeditions are one of the two Options in Soldier of Fortune mode. you need to select the game type in the character screen to have those available. the NPC's for them aren't in story mode, and IIRC you need to finish the storyline at least once to unlock it. Story is fast though.
Can the character level go higher than 20?
Not at this time. They will upon official release, which is currently planned late January 2020.
Wings of Ishmir: my char is constantly sliding around, out of control, as though in quicksand. This has become very annoying. Is there anything that can be done about this?
Not sure about this, I'm having no issues with "sliding". I do get knocked around on occasion when landing. There's a delay after land that allows mobs to hit you, and some of them have a knockback effect. If you have multiple mobs like this in a group, it can easily feel like your being ping ponged around, very frustrating. Personally I think they need to look at the delay after landing with wings...the stun effect is almost a necessity give this deadly delay. If you aren't using the stun, I highly recommend it.
I'm constantly falling off the ledges in SOF. Every blue moon I'll parachute down into town. But, 95% of the time I get stuck in limbo. Then, I have to restart a new game via the Character Screen. Funds are never reimbursed, and my efforts are waisted. Is there anything that I can do, or is there anything being done, about this?
Not sure about this, I haven't experienced this yet. Post it in the bug forums...make sure you seach for the bug first so were not double posting bugs.
My beginners build has been melee, but there are only a few skills, here, that are really useful in SOF. Could someone direct me to a few nice mage skills to try out? And, would I need to carry a staff to use them?
My initial build, that I'm still using for the most part is a melee build based around high crit and moderate bleeds. I leveled with a 2h weapon...it instagibs most mobs very easily. I switched to a 1h+shield combo once I started end game vet levels and so far it's going pretty smooth.
Wings of Ishmir
Anvil of Woe
Last skill is flexible. I used Sovereign shout for ages until I got a Quintet of Sundowns shield, which allowed me to use Bulwark of dawn (very helpful in mid+ vet runs). Emphasis in skill mods are stuns. if it has a stun, use it. Anvil hits like a truck on stunned opponents (make sure you take the passive for it).
Passive tree highlights crit damage, crit change, and then I take the Blood Reaper keystone in warmonger for rend damage (bleeds). Take the shield passives in praetorian for extra defense. Currently stacking heavy armor for the resist all, and it has a nice blend of defense and offense. I also take the nodes for extra stamina points, and use the skill mods for Wings that eliminates the Cooldown. With that and the stun on it, you'll be abel to clear easily with the huge crits on Anvil, and then you can kite with Bleeding edge in tense situations.
This isn't an optimized end game build at any case, but really more of an evolution of the skills I picked up along the way. It has a high movement, fast paced style with big hits that I like, and it can do mid vet levels with no problems.
If you want something slightly different, here's a link to a hybrid caster/melee build by Marty GameDK. fun build, I'm using it on an alt, but it does require a specific unique to play out. But This unique seems to drop regularly...I've gotten it 4x or so since I started playing, which was only a week ago.