HTTPS guarantees an end-to-end encrypted connection (if some server in the middle is decrypting it, you will get a certificate error). You should not proceed if you get a certificate error, of course. Yes, there is a problem with forged certificates, but that is exceedingly rare, and it does not at all relate to open wifi network usage. Your data would be just as compromised in that case if you were on encrypted wifi or hardwired ethernet.
You are correct that many web sites will start out as HTTP and then only redirect you to HTTPS when you go to pay -- this is a problem, and many website operators have recognized this and now offer their entire site on HTTPS (you just have to edit it in the address bar). They may offer their entire site through HTTPS without even realizing they're doing so. But do beware of sites that will redirect you from HTTPS to HTTP after you log in, or ones that mix HTTP and HTTPS content on a single page (most browsers will pop up a message in this case saying "some items on this page are not secure").
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