Which type of reactions are easily performed by alkane and alkene?

Reactant $(A)$ can be

When the hydrogen atom in isocyanic acid $(HNCO)$ is replaced by a methyl group,the resulting compound undergoes rearrangement to form which product?

"$P$" is a hydrocarbon with the molecular formula $C_8H_{14}$. On ozonolysis,"$P$" forms "$Q$". "$Q$" on treatment with alkali under reflux conditions produces "$R$",which on treatment with $I_2/NaOH$ gives a yellow precipitate. Acidification of the solution gives "$S$". The structure of "$S$" is given below:
(Structure $S$: $1-$methyl$-2-$oxocyclopentanecarboxylic acid derivative or similar cyclic structure as per common reaction pathways).
The correct structure of "$P$" is:

In the given sequence of reactions,identify $P, Q, R$ and $S$ respectively.
$CH_2=CH_2 + Br_2$ $\xrightarrow{P} CH_2(Br)-CH_2(Br)$ $\xrightarrow{Q} CH_2=CH-Br$ $\xrightarrow{R} CH \equiv CH$ $\xrightarrow{S} C_6H_6$

Number of $\pi$ bonds in $CH_2=CH-CH=CH-C\equiv CH$ is