>1. Tissue damage which results in losses instead of gains<
Tissue damage that results in losses? Not familiar with that really. Care to be more specific. There are guys who’ve said they have “broken” their penis. There are lots of threads to search on this and the term you want to search on (beyond broken) has departed my mind for a moment.
>2. Nerve damage which results in less sex pleasure<
Nerve damage will result in less sexual pleasure or potentially pain without stimulus.
>Are those results permanent?<
Are what results permanent? Nerve damage seems to take an excessively long time to repair.
>How long can it take for these negative results to develop?<
How long is a piece of string? Depends on how stupid you are, really. If you are not a stupid person then they won’t develop ever. If you are incredibly stupid, you might get them to develop really quickly. Nerve damage might develop over time, especially with hanging, and this might not be immediately noticeable. Rupturing your dick in another way is likely to be a matter of weakening it over time and then giving it a final push, you probably need an excessively high stupidity quotient to hit this target.
Be excessively careful as a newbie. Practise PE in good light. Ramp up your practise gradually and with care. Do not use any of the exercises marked as advanced without having done enough PE to be able to weigh the risks against the rewards (clamping and bending might be considered too risky to do - ever - they have the highest incidence of injury, hanging might be worth considering if you have made no manual gains etc.).
>How easy is it to cause injuries? Are there warning signs like a certain kind of sensation or something?<
There are always warning signs. Read as much as possible and be certain you know what you are doing before you start. PE fools rush in.
>How common are injuries for newbies? Is it something everyone does at first?<
There will always be the chance of minor injury (blowing a capillary or vein wall etc) because to some extent you need to sail close to the wind sometimes but if you recognise when you are doing this you will be more careful, right?
Read everything. Then start.