Re: I have several questions

@WallFlower26 Wow, that'a a lot of stuff... and I don't think I will sufficiently cover all you want to know here, but I can add some info. Hopefully something helps...

1. Can’t you go to Sims houses by pressing on their picture in the Relationships tab? You can’t do that.?
-  You can text, chat, or invite them over that way, though.
-  But no, you cannot go to their home by clicking their picture in your relationships tab. However, they may contact you to invite you over, or to invite you to join them in going somewhere.
-  One of the reasons for this may be that a NPC/'Townie' (Non-Playable character) is a game-generated Sims that cannot be found in ‘Manage Households’ and don’t actually live in the Sims world. Example: The Grim Reaper.  So, if they have no home, you cannot go there. 

2. Why doesn’t the underwear stay to whatever you change it why does it go back to the original sort when you change type of outfit in CAS? How do I change the underwear?
I expect this is as intended since oftentimes players may want underwear, makeup, hair, etc. to be applied to only one outfit, and if it automatically applied to all this may override aspects you did not intend to. I think it would be a great suggestion to have a toggle on each aspect of the outfit/makeup/hair, etc. that allowed the choice to 'apply' to all outfits, though, and also to then go to any particular outfit and 'unapply' that aspect form just that one outfit in that case.

4. Does the ask/spray monster whatever it is only show when clicking on single beds?
Yes. The 'spray monster' interaction is for the parent or certain household sims to have a way to comfort a child sim who has a fear of the monster under the bed at night. They assume you will only have a child sim in a single bed. (Tip: If you want to get around the monster under the bed thing you can give the child a double bed or add a nightlight to the bedroom.)

5. Rahmi Watson in Henford-On-Bagley is a Style Influencer but the clothes she wears for work are rubbish how do I change them? Is there a way to change them?

Since they live, by  default initially in the world, in Henford-On-Bagley, you could just enter the household and change her clothing and save. However, if the clothing you are referring to is her work outfit that may not be an option in her normal CAS... you could try changing her clothing via the full edit mode cheat.
You can also find the Watson Family in Manage Households > Other Households:

6. Rashidah Watson in Henford-On-Bagley already has a love interest - Rahul Chopra - but you can’t go to his house, how do you go to his house? I can’t find him on a map/neighborhood, I swear I saw his Mum show up when I was doing the Watson household.?
See above. And you can also find the Chopra household in Manage Households > Other Households:
However, as you can see in the screenshot below, the Chopra household is 'currently not in world', which means they don't have a home they are moved into that you can visit. You can change this by moving their household into a lot.

7. I really don’t understand how to keep the Eco Footprint green.
-  There is some great advice on Eco Footprint in this Carl's Guide: https://www.carls-sims-4-guide.com/expansionpacks/ecolifestyle/eco-footprint.php
-  There is information here on known bugs in regards to the Eco Footprint (and you can turn it off in game options): https://answers.ea.com/t5/Bug-Reports/READ-FIRST-Compiled-list-of-reported-Issues/td-p/3445209/page/2#Footprint18

8. How/where do I get collectibles apart from frogs and fishes as I know where to get those. How do I get the dirt pile?
I'm not sure I'm understanding your question correctly, but if you mean the dirt piles that spawn, it may vary. For some types of items you collect you may need to collect from current spawned dirt piles that are in the particular world/area you are in, then you may need to leave the area and return at a different time to see more spawned dirt piles with collectibles (ie- Sixam), and for some collectibles (like plants spawning then maturing in the wild) you may need to stay in that world area for a certain amount of time. 
There is a nice Carl's Guide on collecting at this link: https://www.carls-sims-4-guide.com/collections/