Actually, at the time Archangelsk was a pretty big export port (timber mostly), so it's actually possible for ships to be there on non-military business (inasmuch as trade with an enemy can be considered non-military), so again, the Germans are left with a choice, either let the ships pass, or risk angering the British.
Don't forget "cruiser rules" - they basically allow the searching of ships (of neutral parties) and confiscating (part) of the cargo or taking away the whole ship under certain conditions.
Germayn could (and would) send cruisers to teh Barentss ea to intercept the shipping of supplies - GB would (at least initially) NOT form escorted convoys (they did so OTL only in 1917/18 IIRC)
THE Channel itself would be off limits to German ships (UK promised to keep the French coast clear) All Germany could do is try a distant blockade.
Overall I assume that a Russia fisrt would lead to a German success in the war - even if Britain joins later - As long as Britain is out of the war the additional French production is not making up for Brritains neutrality - France was short on men OTL and not so on materiel - - without Blockade the GErmany could also import like France did OTL.
Stopping Russia with 2/3rds of the Army + Austrias Army might take away enough fighting power of Russia to enable a negotiated peace early - if not I don't see a better performace of Russia.
On the Western Friont Germany likely can hold off French attacks - which will be costly. Even if Britain joins (later) the damage to the French army will be done.
Italy also might not join - or at least not as soon as OTL.