When I send or receive IOTA, I first see the transaction in my wallet as in pending. As the time pass, the transaction is still in pending. I have to wait half an hour minimum to do the reattach. And I have to try to do the reattach 5 times before I can suppose the transaction failed. That means that for about 2 and a half hours I can't make any other transaction.
The best way, to get your transaction through is to run your own full node on a seperate Server. This makes transactions faster and gives you more control. However, if you don't run your own full node, I recommend you to check https://iota.dance/nodes and choose the first/highest node (which is the public node providing the best chances, because of the lowes ping and best connection) within your light wallet. This is done by selecting "Tools > Node configuration" and setting the before picked node. Then you reattach the transaction after every 30 min, not in higher frequency.
In Iota no double spend is confirmed. If you didn't do a double spend, it is very likely that your transaction gets through, as soon as the chosen full node is able to. So actually you are the one who is responsible for your transaction getting through or not. The chances are depending on the full node you are using. So we recommend each user to have his/her own on a seperate Server.
The android Wallet works similar. You can choose the node right before reattaching. Changing it afterwards has no effect on previous reattachements.
To your initial Question. Yes it is possible to send a second transaction from the same address on android wallet. To avoid double spends from the same address I recommend you to use the light wallet and not the android wallet. If you send a second transaction from the same address, you should make sure, that the address is empty afterwards and get it on another address. Because the private key of the double spending address is unsafe now.
A transaction has not failed even if it does not confirm. You should never make a new transaction, but replay the first one until it confirms. The only time a transaction will not confirm, is if the signature is invalid (not possible to do with wallet), or if the address (or one of them) you spend from have a lower balance than the transaction requires.
Every time you sign a transaction, you expose parts of the private key for all the addresses used in the transaction. Even if the transaction stays pending, the key is exposed, and a new TX will make it wors by exposing even more of the keys.
And to the question about sending again. The features of the IOTA wallet changes a bit every time, but you cannot use money you don't have, so if you have 6 Gi in first address, and first tx is to send 3 of them somewhere, all 6 Gi is locked in that tx, and the remainder will not be available before first tx is confirmed anyway. And if the remainder is 'on its way' to an address, you should not sign any other tx out from that address wither, as this would cause you to receive IOTA to an address you have signed from when the remainder arrives.
In short - IOTA-wallet should not be used to make more than one transaction at the time. Make it confirm by reattaching/changing nodes etc before attempting to make a new transaction.