I have postural orthostatic tachycardia syndrome and my blood pools in legs and hr rises, will midodrine allow a lot more venous return and reduce it?

Theoretically, yes. Since midodrine causes constriction of the arterioles as well as the venous system, it is anticipated that venous return will increase and blood pressure will bee higher than the pre-treatment level.
Yes. Midodrine is an Alpha agonist, so it will cause constriction of the blood vessels. This effect would be expected to reduce the dilation/'blood pool' in the leg blood vessels, therefore resulting in an improvement in symptoms of pots. Some people will also wear prescription fitted socks to provide external pressure to keep the blood from 'pooling'.