Alpine Route to and from Toyama

Hi, I am looking to do the Tateyama-Kurobe Alpine Route in 1 day, from Toyama. Travelling in a group of 5. Thank you for helping to answer the questions below!

1. From Dentetsu Toyama station, do I need to pre-book the train ticket to Tateyama? And which website can I use to pre-book the ticket?

2. After completing the Alpine Route, is it worth stopping at Ogisawa, Nagano or Shinano-Omachi? I am planning to stop at Ogisawa as it seems it will be a full day already.

3. If we stop at Ogisawa, how do I get back to Toyama? Or is transport from Shinano-Omachi easier? I am looking for the most convenient way back to Toyama - so is there a bus that we can take from Ogisawa direct to Toyama?

1. From Dentetsu Toyama station, do I need to pre-book the train ticket to Tateyama?

No, it is not possible to reserve seats for these trains.

2. After completing the Alpine Route, is it worth stopping at Ogisawa, Nagano or Shinano-Omachi? I am planning to stop at Ogisawa as it seems it will be a full day already.

Ogisawa has nothing to see.

Shinano-Omachi has a small number of attractions:
http://www.kanko-omachi.gr.jp/en/

Nagano has a variety of interesting attractions:
https://www.japan-guide.com/e/e6000.html

If we stop at Ogisawa, how do I get back to Toyama?

Travel back the Alpine Route or carry on to Shinano-Omachi from where you can take trains via Itoigawa back to Toyama or take the bus to Nagano and take the shinkasen back to Toyama. The trip via Nagano isusually the fastest and easiest:
https://www.japan-guide.com/e/e7552.html