Does the book say how the Li- was formed initially?

please rephrase that

with regards to HCl(aq) 40% is the highest I`ve known and that was sold as a Technical grade reagent in ampules to be dilluted in 1 ltr of deionised water.

you can get stronger HCl(aq) but not at standard RTP, so it`s basicly useless unless you have something specific in mind, and the related equip to handle it

 

with regards to Ammonium Ions, have a look at some of Lewis` work, he spent nearly a lifetime with experimental studies around this compound, I`de consider him a excellent source

I have had much success with a solution of silver sulfate and sodium hydroxide. Even with the hydroxide you wont get a huge amount of silver ions but there will still be plenty for electroplating.