Importing image to InDesign - jpeg-format and PPI

Yes, all will be ok - as long as the JPEG was saved with Maximum Quality 12 in the settings.

As @Diane Burns mentions - it would have best to leave the images as PSD or TIFF.

Or if you had text layers/vector masks/vector shapes - then saving as PDF is preferred.

300 PPI in the effective is completely fine. I wrote an article here that explains some of what you asked
https://creativepro.com/high-res-image-look-low-res/#:~:text=InDesign%20sometimes%20uses%20a%20low,access%20via%20View%20%3E%20Display%20Performance.

Finally - 300 PPI is a false target to aim for. Where it's a good round number.

It's only true for things being printed in Lithographic Print - where the LPI (Lines Per Inch) is set to 200 LPI.

This is a technicaly printing guide for Lithographic Print.

1. Mostly LPI is set to 150 LPI but it depends on the output.
2. Newspapers use a lower LPI - usually around 80-120
3. Magazines could use LPI of 120 - 150
4. High end magazines could use a LPI of 150-200

Regarding no. 1 above - this would mean a DPI of 225 would be sufficient for print

no.2 - newspapers - you could use a DPI between 120-180

So the 300 DPI (or PPI) rule is not set in stone (little litho joke there).

And it only really applies to lithographic print for magazines/books/newspapers etc.

For anything outside of digital printing like

Screen Printing/Digital Printing/Large Format Printing

You could use a lower than 300 DPI setting and it will look perfectly fine.

That's because these types of printing use a different method of printing. 

If it's posters that are viewed from a distance - you don't need 300 PPI. You could use 150 PPI.

If it's a Billboard you could use 30 PPI.

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All of this means you need to talk with your print provider and supply them with what they need and recommend.

Regarding question 2, the Effective PPI of 300 is fine, if you have some over this size, it doesn't matter too much (or you can resize them first in Photoshop).

You will, presumably, be offering the printing company a PDF for printing, it's important to ask them what kind of PDF they want, for example PDF/X-1 or -4, normally they want single pages (not spreads) crop marks and tick Use Document Bleed Settings.

Regarding question number 1, you can just import the .psd files, no need to convert to .jpg