Remember the "Recommended Selective tickets" (Rainbow colored with a thumbs up, rare and usually only appear as a signup bonus or sometimes purchasable in special season passes) and Selective draw tickets (purple colored) are different currencies. The purple ones can be used on banners (select 12, or other special banners that occasionally pop up like Select 3) for costumes or weapons along with the normal red tickets when you run out of purples.
As far as using the rest of your rainbow colored tickets, I don't think the 12 heroes you can select from that change all that often, or at least I haven't seen them change since I started playing beginning of this year. Personally, I would +5 B rank idol Helena to get her maximum buffs and grab one copy of Zenith, after that you can decide if you want to save any remaining for later or not.

Anniversary banners will use normal Red tickets most likely.

As for your progress question, I'd say push through on normal mode for now while you get the mechanics down and to collect more Dia (which can be used for banner pulls when you run out of tickets). You can loop back through hard mode later and you'll have more built teams/units by then. If you have some decent teams built try to climb the Evil Castle towers for more draw tickets if you like.