1)
かたち is the most common for single kanji. かた might be possible, but けい is rare. As for a suffix, it's basically "on'yomi + けい", "kun'yomi + かた/がた" (e.g., 正方形[せいほうけい], 菱形[ひしがた]). There are exceptions, of course.
2)
Yes.
3)
Those each numbers you saw on the graph are AVERAGE 初婚率 of each five ages, for instance, "44.88 for 20-24" means the average of 初婚率 of 20, 21, 22, 23, 24 years old women is 44.88人/1000人. Thus, TOTAL 初婚率 of 20-39 are approximately 650(130×5)人/1000人.
4) a)
「○○スル」 refers to "-suru verb stem". Katakana is used for difinition in this case.
b)
つく can mean "word that CAN be atteched to 未". Thus, 「未」がついた言葉 only refers to 未解決, 未完成 or 未使用, whereas 「未」がつく言葉 can be used also for 解決, 完成 or 使用.