If you're planning to play Magicka then no it's not really worth investing in active weapon skills as they all scale off Stamina and Weapon damage, whilst all the Magicka ones scale off Magicka and Spell damage.

The last update changed skills so they will now list their costs type in a colour relating to Magicka or Stamina as well so you can see which ones will be effected by what (it was always listed before but required reading rather than looking for coloured text).

If you're going DPS you may want to invest in dual wielded swords and some of the passives from that skill line as they add a base damage benefit to the base damage of your weapons (which in turn becomes magicka/spell damage).

You should be able to run all the skills required to play both a healer and a DPS whislst also still being able to use all crafts. This will however be a heavy SP investment and you'll need to finish all the games main stories, find all Skyshards and maybe partake in some PvP for additional SP. I used to be able to play Magicka tank and DPS with all crafting on about 250 SP, however there are more places to invest skills since that time (and more sources of SP from DLC).

To be dual spec you're mostly going to be changing your gear but have the advantage you'll use light amour for both. The conflicts might come from skill morphs however if one morph is better for DPS and one better for healing. Lastly once you start using champion points you may need to invest these in different areas when switching between DPS and healer, the costs for these are not great (3k for a full reset) but something to be aware of.

I can't help with specifics for builds as I don't actually have a Magicka Templar (yet) though sorry.