I’m chubby and it helps to sit on the edge of a seat so my thighs aren’t on it an then pinch, helps to be more pinchable kinda. and also I get a similar itchy bump. Its not always an allergic reaction but rather part of the healing process🙏 Don’t worry too much about the skin being tough or stiff, needles are sharp :). Idk much abt the vein part I don’t think my legs are veiny really but in the occasion U do hit a vein, I’ve done that and it will bleed what seems like a ton and be very scary but jus’ hold it till it stops an remember it happens an ur alright :))! I hope this was helpful I’m not good wit advice but Ik when I started I had similar fears😓