So, a couple of things aren't necessarily correct here. You can hit Tier 12.5 with an entire week of playing. This is doing every single daily mission, every single weekly mission, and all tier missions. I know this because that's where I am and that's all I've done.
This means with the Season being four weeks you can get 50 tiers. Now, assuming that it makes you go back through each tier even if it doesn't have a secondary award, you can get all but 10 without paying a dime for more tiers. (I also noticed one tier had a third reward, so idk how you would ever get that normally.)
That being said, can some one post proof of their alts having stars available AFTER they've completed daily missions on their main? I have an alt and this is not the case. You can complete the missions on ANY character, but I believe only ONE time. However, if this is true, then that would mean you could do 150 Tiers for a 4 week season and, thus, would never need to pay for tiers. (This is why I find it hard to believe you can do extra challenges with other characters, there's 0 incentive to pay at that point.)
@GM-Deynger would you mind confirming alts ability to gain battle pass tiers?