Bonuses from Spells
In general, spells that give a bonus to Attack, Defense, Soak, and the like don't stack. Use the highest bonus that applies.
Aegis of the Hearth
In the Saga of A Family Affair, Aegis of the Hearth lasts a Season, not a Year. It is a powerful defense that can be used in times of crisis, but is prohibitively expensive to keep up year-round.
The reason for this is that it's quite a story-killer. Aegis of the Hearth prevents ghosts and faeries from visiting the covenant, which makes it harder for the storyguide to do cool things.
Wards don't need to penetrate Magic Resistance.
According to the Ars Magica FAQ, wards don't keep out a supernatural creature unless they penetrate its Magic Resistance. This is not mentioned explicitly in the rule books but seems to have been assumed by the Fifth Edition author.
One could make an argument either way. The argument that wards need to penetrate is that all spells need to penetrate and there is not an explicit exception for wards. The argument against them needing to penetrate is that the spell descriptions (such as Ward Against Faeries of the Waters, ArM5 p. 124) say how the spells work, and they only mention Level and Might Score, not Penetration.
We think the latter argument is stronger. If wards need to Penetrate, they are seriously under-powered and the spell guidelines should be revised. The lower-impact fix is to play the rules as they were written (though perhaps not as they were meant to be written).