Based on your xray, I could see a lot of issues here. So lets begin

1- You have generalised periodontitis, meaning you have gum problems, you seriously need to improve your oral hygiene. Your dentist should focus on it immediately.

2- There is generalised loss of bone between your teeth, thus indicating periodontal diseases aka gum disease. Elaboration of my 1st point.

3- Now lets address your big red arrow.

a- Your last tooth might have developed secondary caries, for this you need another type of xray known as PA view, which focuses closely on 2 or 3 teeth only, unlike your current one which is an OPG.  There are several reasons why secondary caries develop, so I wont go in their details but it doesnt reflect nicely on who ever did your filling in the first place.

b- There is widening of space between bone and roots of your teeth, its known as widening of Periodontal ligament space or PDL space. Few common reasons for that are, Occlusal trauma (from over filled filling), Periodontal disease and lastly Pulpo-periapical lesions. In your case its most likely overfilled tooth and presence of gum disease.

c- The dark spot you see on the xray is very diffused, meaning its borders are not well defined hence the infection is not active and might be transient, a PA xray would make things more clear here.

Did your dentist conducted pulp vitality test, meaning did he touched your tooth first with hot stuff, then cold to see if the tooth is vital (alive). This test is very important to establish if the pulp had developed necrosis hence needs to be removed via RCT, if not then you need a new dentist.

Here is my recommendation, get a second advise from a more competent and experienced specialist before getting RCT from your current dentist, I will refrain from making any further comments on the competency of your current dentist. Best of luck.

1- Yes, its very clear because of the bone loss levels shown in the xray. Scaling is not itself a solution, you need to improve your oral hygiene. You need to change your brushing technique, learn to use floss and include use of mouthwash. In the beginning Dentist might advise you a medicated toothpaste. Scaling is a critical part of treatment regime, but its effective in combination.

2- I can not say why your dentists didnt advise Oral Hygiene Instructions to you, periodontitis is as clear as day light here. You might also be suffering from bleeding gums and develop bad breath from time to time. If you are a smoker, cut down on number of cigarettes.

3- a-The 2nd opinion dentist was stating that since there is no 3rd molar, the caries could have developed from the far side, again if that is the case the reason is your periodontitis. In healthy gums, the root material is not exposed instead covered by a combination of bone and gums. The material on the root is not as resistant to caries as the one on the top of tooth. I simplified tooth anatomy for you, the interaction is far complex.

b- Yes, ask your dentist on how to properly brush your teeth, and the frequency of brushing. You might need scaling and root planning (a special type of scaling) Refer to point 1.

c- Vitality test consists of hot and cold test, some times electrical stimulus is also used. If your tooth is sensitive to touch, aka Tender to touch, further probing is required to establish the cause for it.

I do not know what discomfort means here, its a very subjective term. There shouldn't be any pain or tenderness, if so antibiotics might be required to control the infection.

Once RCT is performed, the tooth must be crowned as once the pulp is removed tooth becomes very brittle and could crack. Extraction should be the last option, when everything else fails. In your case you need to improve oral hygiene.

Again I wont comment on dentists competency, but if they keep on missing developing and worsening of your gum disease, go figure.