Let me try and unpack some of your questions:
I am arriving in Porto on the 9th in the evening and have a flight back to Manchester from Porto at the 24th of Sep.
Do you need to get to Porto on 24 Sep to catch an early flight, or will you be able to catch an bus from Santiago to Porto and arrive in time to catch your flight? One way or another, you will need factor the trip into your schedule.
how much I could walk every day (I am in good health condition and train every day but still!) and how could I plan each day/stage, do I need to plan it?
I recommend having an outline plan, but without a fundamental like how many km you reasonably walk each day that might be difficult. The outline can work a couple of ways. If you know what maximum distance you want to walk, you can then work out how many days you need. Or if you already have a time limit, you can work out how far you need to walk each day.
You indicate that you have 14 clear days, 10-23 Sep. If you spend a couple of days in Porto at the start, and need a day in SDC at the end, that is 11 days to walk. The Littoral is about 270 km, suggesting you need to walk an average of just under 25 km each day. To achieve that, you might contemplate having to walk some days of 30 km or maybe even greater distances. If you don't know yet how far you can walk, even the shorter days might be too far.
Even if you forego the days in Porto, you have 13 days, your average will need to be 21km/day, and you might expect to have to walk some days of around 25 km. Can you do that?
They could, unless you need to rely on a Xunta or other albergue that doesn't take bookings. It was convenient for me to stay at O Pino when I walked in 2022. It didn't fill up, but that is no guarantee, and arriving as early as practicable often means taking a short day, something you mightn't have the luxury to plan for.
I also wondering if I need to stay at Santiago for a night or can get all stamps same day of arrival? (found so many confusing posts)
The current process has been considerably improved, and the waiting times of an hour or more are no longer being reported on this forum. I don't try and push my last day, and have taken to arriving mid-afternoon, and had ample time to get my Compostela on the day I arrive. Older posts from before the current system was put in place would express greater caution here, and rightly so. Look for more recent information, basically from members who have experience this year, not anyone whose experience is earlier than that. There were still long queues when I walked in 2022 that were completely gone this year.
Also maybe some recommendations about staying in Porto? I never been to Portugal and thinking of staying for a couple of days prior to my camino but not sure If I would have time , especially with a fact that I will need to travel back to porto at the end.
I would put my priority into walking, and if you then have time, consider any sight-seeing after that.
I want to start coastal but then go to central route after Vigo, how do you think if this a good plan?
There are other ways of joining the Central route, but if you walk to Vigo on the Coastal/Littoral, you really only have one choice from Vigo, and that is to walk to Redondela.
ps the questions are rhetorical. You need the answers so you can do your planning. Having said that, some members might share a walking schedule they did that achieved this in the time you have available. However, given you don't know how far you can walk, my thinking would be to start with shorter distances and push those out as you get some experience with what you can achieve. This would mitigate against booking accommodation any earlier than a night or two beforehand as you better understand your walking ability.
