Speed App linked as your LN Address on Stacker News = to receive Zaps directly on Speed.
To buy CC, you pay an invoice with Speed App (so you send sats).
The seed phrase you're talking about is just to unlock the internal Stacker News interface (SN's custodial wallet), not the Speed App seed.
So, regarding your Chrome issue:
When you open this interface with the seed (the 12 words of Stacker News), Chrome thinks it's a login/password and only saves the first word.
This isn't a problem for Speed App, because this seed isn't Speed App's seed; it's just the seed for Stacker News' internal wallet.
But do this to be on the safe side:
After unlocking, go to passwords.google.com and delete the "stacker.news" entry, which has one word.
Disable autosave when you type the seed, so it won't save it anymore.
You don't need to keep this Stacker News seed if you receive everything on Speed App. It's just a temporary wallet for buying CC.