Hi
1. To avoid to many people, July is a good month.
2. There are bridges on some rivers, but not all.
3. I would recommend to take the bus to Bygdin and then take the boat to Fondsbu. That is a good starting point. For bus:
http://www.nor-way.no/
For boat:
http://www.jvb.no/bitihorn/
.
4. You have picked very popular routes in Jotunheimen
A nice trip is Fondsbu – Olavsbu –Skogadalsbøen-Olavsbu (choose a different route) – Leirvassbu – Spiterstulen – Glitterheim and then to Gjendesheim, where you can take the bus back to Oslo. There are, of course a lot of other route options.
5. Both Galdhopiggen and Glittertinden are easy to reach. Spiterstulen is a good starting point for Galdhopiggen. If you walk from Spiterstulen to Glitterheim, one of the route are over Glittertinden. There is not so crowded on Glittertinden.
6. I have never been in Trollheimen, but I assume there is not that many people there
Hopefully the input from me help you in your planning.