[RPG] How to Constructs use Magic

First question: trigger

The character needs only to have at least one level of a class which can cast the spell. In this case a first level wizard will be able to trigger any wizard spell.

Second question: completion

From the SRD: Scrolls

To have any chance of activating a scroll spell, the scroll user must meet the following requirements.

• The spell must be of the correct type (arcane or divine). Arcane spellcasters (wizards, sorcerers, and bards) can only use scrolls containing arcane spells, and divine spellcasters (clerics, druids, paladins, and rangers) can only use scrolls containing divine spells. (The type of scroll a character creates is also determined by his class.)
• The user must have the spell on her class list.
• The user must have the requisite ability score. If the user meets all the requirements noted above, and her caster level is at least equal to the spell's caster level, she can automatically activate the spell without a check.

This means that the character must be a spell caster of the correct type (arcane or divine): so no a fighter will not be able to use a scroll.

If she meets all three requirements but her own caster level is lower than the scroll spell's caster level, then she has to make a caster level check (DC = scroll's caster level + 1) to cast the spell successfully.

In this case: yes a first level wizard will be able to try with any arcane scroll (if he has the required Int) but he will have to perform an appropriate caster level with the following consequences:

If she fails, she must make a DC 5 Wisdom check to avoid a mishap (see Scroll Mishaps). A natural roll of 1 always fails, whatever the modifiers. Activating a scroll is a standard action (or the spell's casting time, whichever is longer) and it provokes attacks of opportunity exactly as casting a spell does.

Use magic device

You can use this skill to read a spell or to activate a magic item. Use Magic Device lets you use a magic item as if you had the spell ability or class features of another class, as if you were a different race, or as if you were of a different alignment.

This allow to override the first condition: even if you are not of the correct class (or have the wrong alignment and so on) and you have the use magic device skill you can do a check and try anyway.

So in this case if your 20th level fighter is trained in use magic device: yes he can use a scroll

No, it can't. Animate dead and similar spells typically have a duration of instantaneous. This means the magic is only present at their creation (and for more powerful spells/spellcasters, an aura may linger for a time). After that, the negative energy has already infused the creature and made it undead - this is a "natural" (stretching the term quite a bit) state for an undead creature, and requires no ongoing enchantment to keep it functioning.

A lot of confusion on this issue stems from the fact that the only way mortals (read: your PCs and the vast majority of NPCs) can manipulate positive and negative energy is through magic. But a dead body infused with negative energy in other ways can become undead without any "magic". A deity capable of manipulating negative energy directly can just push the energy into the creature. A place where lots of dark things have happened can naturally attract negative energy, and people who die there may rise as undead. Some undead can "infect" living targets with negative energy carried by a natural vector, like a disease or poison (like plague zombies). None of these require any magic to create the undead.

Basically, animate dead just "starts the engine", the creatures can "run" without any more magic.