Short Lived Magic -

Regarding this flaw, does any group allow a magus to take spells with +1 mag to enable D : Sun?

If not, then a magus with this flaw can never cast rituals with D : Year!

Or does that violate the point of having a flaw?

A magus with this flaw must cast a spell with D:Moon to get the benefit of D:Sun. And, yes, it is the point of the flaw, that (all) their spells are shorter in duration compared to a caster without this flaw. And, I don't believe you can add an +1 magnitude to go for a duration beyond a Year.
I would not take this flaw if I expected to cast a lot of rituals with D:Year, as it becomes a bit more than a minor flaw for such a character... Definitely not a flaw you want your Aegis caster to have!

In my group we found this to be debilitating enough that it was upgraded to a -3 major flaw.

I meant that if you want a spell to have D:Sun you learn it at +1 magnitude so wall of proetcting stone (CrTe25) must be learnt at level 30 if you want your wall to last until sunset/rise; rather than diameter!

Maybe you could allow the above for a -1 flaw; -3 flaw means perhaps you can't modify your spells is that way. As was mentioned this makes it a particuarly bad flaw!

You have to increase the magnitude by +1, yes. To change the duration to Moon. You can't simply add +1 magnitude for every spell to increase the duration. If someone with Short Lived magic wants a spell to last for Moon duration they have to invent a version with D:Year, which makes it a ritual spell. It is probably for this reason Heaven Thunder Hammer thinks it should be a major flaw, and I'm not in strong disagreement with his position.

One solution would be to design every spell as Concentration (for +2, for the short-lived caster), then use a magical device to use Maintain the Demanding Spell (created by someone ELSE) to get it up to Moon.

But yeah - that does seem to be more disadvantageous than a minor. Of course, if you're only casting Instant/Momentary effects, then it's not an issue. So I suppose it depends on what kind of spell list you have...

Actually that wasn't the only reason, my group is big on "buff" spells, and this player has to reinvent every spell separately from the other players and can't use a lab text.