Squarespace's Code Injection panel is limited but real — it's a header and footer field that applies across the whole site, no separate publish step required beyond saving.
Not always — Squarespace has historically restricted Code Injection to its higher-tier plans rather than the entry-level plan, so check your account's current plan details if you can't find the setting.
Yes, sitewide Code Injection applies to every page template, including product pages, unless Squarespace routes checkout through a separate hosted flow.
No — Code Injection is a site-level setting independent of which template or design you use, so a template switch does not remove it.
The script loads with defer and is small, so its effect on speed scores is minor — far smaller than most embedded fonts or image galleries already on the page.