Thanks for these questions. I hope I can give useful answers.
1.) Is it possible to add the strength sessions from the transition period to the base phase as I feel they could be benefitting for me. But I think then I will probably not be able to handle the more strenuous running workouts as well as I do without. Also I thought I read in Training for the Uphill Athlete that in the base phase there is also some strength training recommend. Why is it left out in this plan? You can add them in but if they are negatively impacting the more intense running workouts you will need to shorten or drop them all together. The reason there is no strength training in the base period is for exactly the reason you state. For those with little running background the running intensity will be enough strength stimulus. Better to under train than cook yourself by trying to do too much. The Uphill Athlete book is geared towards runs with much more vertical gain and loss. For those, more strength can be beneficial.
2.) Is it possible to extend this plan? Which weeks would be best to be repeated? Yes, for most people using this plan extending the first 8 weeks should be the priority. If you have enough time then extend the base period weeks. This is highly personal but these are general rules.
3.) Which training plan would you recommend for after I finished with this plan? I have two events I would like to take part in about 18 (25 km/750 hm) and 28 weeks (39 km / 2000 hm) after the plan is finished as they all seem for a little bit longer distances. The best follow on plan if this one goes well for you is to move to the Luke Nelson Intro to Ultra plan.
I hope this helps.
Scott