Britain had no need for conscription before 1916. The army couldn't cope with the flood of volunteers. There were shortages of everything from Artillary to boots. Men had to wait months after volunteering before they could report for duty, and faced the scorn of their neighbours who thought them cowards while waiting. Also Britain had no tradition of conscription for the army, and the closest equivelant the navy's press gangs had ended generations earlier.
Putting extra troops into the western front would have been pointless as in 1915 there was no way to break through the German lines and restore movement to the campaign.
To launch an invasion of the Austro Hungarian empire from the Adriatic the entente would first have to bring the A-H Navy to battle and defeat it. The A-H navy though small was perfectly adequate for it's role of defending their coast, and would have been a match for the Ententes Galipolli force, having more Dreadnoughts, plus Pre Dreds, Armoured Cruisers, Destroyers and submarines. While that force exists no landings can take place. Despite Entente Naval superiourity that force remained viable right to the end of the war.
Of the major Central Powers the Ottomans were the weak link not the A-H in the Adriatic.
Galipolli failed not because of it being stategically flawed (which it wasn't) but because of poor planning and a lack of co-ordination between the Naval and Army elements.
Yes, the did. First, by drafting the UK avoids shipyard workers, ammo plant workers, coal miners and other critical occupations from joining, and drafts more clerks and lower value workers. This means more production of armaments and a faster growing Army/Navy.
Now I also believe they could have built up their forces faster, but even if I happen to be wrong on this point, a better, more systematic use of manpower helps. The idea of the draft was debated months before it passed, and I have a hard time believing that in the months leading up to the draft, there was no need for extra manpower.
The A-H navy could have been defeated, also Serbia did not fall until late 1915, so the UK could have landed at a friendly port. The same roads that evacuated the Serbs army, can allow British troops into attack Serbia. Attacking Germany in France was the best idea, Adriatic second best, and Gallipoli was a distant third. This has to do with the personalities, Churchill like to take risks and like the big risk/big return bet. He did not have the personality that favored the better, low risk, guaranteed average return option of France.
History proves you wrong on the Ottomans. Germany signed the ToV. A-H fell apart. The Ottomans fought the Entente to a negotiated peace. Logistics is the key. In France, it is easy to supply. A-H would have been more moderate difficulty. The Ottomans required building railroads and infrastructure, and was the hardest option. France and the UK came close to breaking the German lines in 1915. With an extra 16 divisions, they have a good chance of succeeding, and even failure will halt the German drive into Russia months earlier than OTL.
Gallipoli failed because amphibious operations are much harder than attacking in an area that has good ports, good railroads, and flat terrain. Flanders was a much better option.