1. IMO, not worth it. Yes it will work with your HRM strap and GSC10
2. Yes. However, limiting the number of course points to ~1500 or less will speed up the 705. Use TCX Converter and/or divide longer routes into two. I've also found that using some OSMs speed up calcs by almost a factor of two, but introduce other issues.
3. Don't know.
4. Don't know. But I find it easy to estimate based on miles to go and typical avg pace somewhere between ~3-4 minutes/mile.

You may already be aware of the official user's forums which can be useful for questions like these. https://forums.garmin.com/forumdisplay.php?20-Cycling