The case for a German forestalling invasion is worse than unconvincing, it's got boring too.
Attrition,
You are misrepresenting the real issue here, which is whether Stalin was planning to attack Germany at some time of his own choosing, ie whether his planning was independent of German actions. It is not restricted to the issue of whether Stalin was planning an attack specifically in 1941.
Much verbiage has been uttered to the effect that as of 22 June 1941 the Red Army was not strong enough to launch a first strike against the German forces massing in occupied Poland with any realistic chance of success. But that is not the essential point. There is a lot of evidence that Stalin was building up the Red Army with the aim of making it capable fighting a successful war with Germany by 1942.
The essential issue is what Stalin intended to do with the Red Army in 1942, once the program of upgrading and re-equipping it had been completed. Did he intend to just sit back and wait for a German invasion, confident that the Red Army was now strong enough to repulse it? Or did he intend to attack first, to secure the advantage that accrues to the side that strikes first against an opponent that is not superior in strength?
The fact is that we do not have enough evidence to decide one way or the other with any degree of certainty. That is why historians are still debating the issue.
But there are indications that Stalin changed his plans once it became obvious that Germany was preparing to attack in 1941, ie before the planned strengthening of the Red Army had been completed, and that he was considering launching a first strike to pre-empt the German invasion. The Timoshenko-Zhukov pre-emptive strike plan is a clear indication of such thinking; the extant rough draft of the plan clearly states that its aim was to attack the German forces in the midst of their observed deployment and thereby catch them off balance, increasing the chances of success.
What is not known for sure is whether Stalin decided to implement the T-Z plan, and whether the actions of the Red Army in the days leading up to the German invasion were part of such implementation. Some of those actions do seem to resemble preliminary moves prescribed in the T-Z plan.
However, there are also indications that at the last minute Stalin had realised that it was too late to try to pre-empt the imminent German attack, and decided to pull back and try to adopt a defensive posture. One such indication is the Soviet withdrawal from Lithuania which began on 21 June, ie before the start of the German invasion, and which triggered an uprising by ethnic Lithuanian units in the Red Army.
All we can be reasonably certain of is that, after the fall of France in 1940, Stalin calculated that sooner or later there would be an armed conflict with Germany, and his only realistic options were either to initiate that conflict himself at a time of his choosing, when the Red Army had become strong enough to be confident of success, or leave the initiative to Hitler and simply wait for Germany to launch a first strike, which he thought it could not do while it was still bogged down in a stalemated war with Britain.
