The blending calculator puts your 100C viscosity with the 75/25 blend t 16.25 cSt, which is borderline between a 40 and a 50 weight. The W number may remain a 10 but certainly towards the 15. Hard to say unless we know the base oil viscosities of both oils.
Yes, you would get about a 15 cSt 15w40 with a 50/50 mix of 10w30 and 20w50 Maxlife. I figure the W number goes to 15 simply by the 50/50 splitting between 10 and 20, crude, but probably about right.
You can mix Synpower with Maxlife. I like Maxlife Synthetic better for the 30 weights as it is more robust (higher HTHS) among other things like moly, and meets ACES A3. For 40 and 50 weight the Synpower is rated quite well, meeting ACEA A3. Since the MSDS for Synpower 20w50 says it's PAO, Synpower 20w50 would be a great one to mix with the 10w30 Maxlife to get a 15w40.