Is 5 mg saxagliptin equal to 100mg sitagliptin?

Approximately yes. It may be approximately but no comparitve studies have been done. Sexaglyptine sold under brand name Onglyza (saxagliptin) has staring dose 2.5 mgm and maximum dose 5mgm.And sitaglyptin sold under brand name januvia starting dose is 50 mgm and maximum dose of 100 mgm daily.
Yes, on DPP-4. The standard dose is 5 mg for saxagliptin and 100 mg for sitagliptin for inhibition of the enzyme dpp-4. The two molecules are very different and might have different effects. Sitagliptin has many fluorides and could interfere with iodine metabolism in a situation of iodine deficiency, for instance.
Yes. Although, these are two different chemicals, they belong to the same class of anti-diabetic medications. Roughly, the doses presented are equivalent and can be interchanged.