[Wereboar]

2. Can someone recommend a party that is challenging but not impossible (practically speaking - I don't have a free month-on-end to do solo spell-less bishop for example) at all stages of the game at normal difficulty?

No.
In normal difficulty, you skills increase quite fast.
IMO you can't have challenging late game with this setting (maybe unless you go for very unusual class combos, which could be quite hard at start)

3. I've found the gimmicky classes not too great (bard and gadgeteer) - is there something that makes them indispensable in anyone's opinion?

Bards have some very nice spells quite early (shrill sound, insanity, haste) and use only stamina to cast it.
A bishop that casts insanity every round will be out of spell points soon. For the bard, just cast stamina on him.

5. I usually train stealth and shield skill in the lower monastery if I have patience for it.

I did the stealt training one (with the fighter/bishop duo). Makes the game far too easy IMO.

A couple of times, for no apparent reason, I have run into groups like: 7 higardi highwaymen 4 caustic vines 6 higardi highwaymen 3 cultists

It just happens sometimes that you get a group of monsters (or two) at a slightly higher level than yours. Highwayman may be level 7, the vines level 8 - just above your party.

cultists! my guys haven't even learned soul shield or element shield at this stage!

But IIRC they are very easy to kill with say fire ball (about 15 HP), and don't have dangerous attack spells (when alone).

You should try to divide them into several fights.
Advance slowly, and when you see a group of monsters (before being in fight range), slowly walk back and drag them with you. Then wait for them at a point where you are protected by terrain (the often won't follow you into close spots, so engange combat just outside and walk back during the first round of combat)