Questions about Seraph stacking (and stacking in general)

This new double tear build on Ezreal consists of buying and fully stacking a Manamune to turn it into a Muramana, and then buy the next tear or tear upgrade and stack that one too.

Manamune will give you 5 mana on mana expenditure

Muramana's passive consumes mana on basic attacks (3% of current mana to deal double that amount as physical damage)

Seraph's Embrace grants 8 mana on mana expenditure

While in the practice tool (with a Muramana and an Archangels), I noticed that when auto attacking, I'm actually on only getting 5 mana, while using an ability gave me 8 mana. Is this a bug, or will it use auto attack mana value from Manamune automatically..? Because I'm assuming that if I use an auto and it costs mana, it should give me 8 stacks towards Archangels no? It's mana expenditure, otherwise it wouldn't stack in the first place, right?

Also, is it possible for a 'max stack' option? Or just the ability to buy fully stacked items, like Muramana and Seraphs? Would be so convenient ><